Question: Who is the artist that sang the song "I'm a believer" at the end of Shrek?
About the article "Shrek", section "Music".

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Donkey in Shrek films.
Donkey first makes his debut at a sale of animal or mythical characters from beloved fairy-tales being sold to the evil Lord Farquaad's knights.
An old woman attempts to sell Donkey, but magic pixie dust accidentally is unleashed upon him from a caged fairy, thus giving him the temporary ability to fly, he flies off saying "You might have seen a housefly, maybe even a superfly, but I bet you ain't never seen a DONKEY fly!".
However, the spell unleashed upon Donkey then breaks and he falls to the ground, he is chased by Farquaad's knights into the woods, where he meets Shrek.
Shrek is at first reluctant to meet the talkative Donkey, who follows him around, but Shrek does not consider Donkey a friend until the end of the film.
Donkey also follows Shrek on his quest to find Princess Fiona, whom Lord Farquaad wants to make his bride.
While passing a mountain guarded by a vicious, fire-breathing dragon, Donkey tries to talk the dragon guard into allowing him and Shrek to pass and compliments the dragon, whom he discovers is a female, and the dragon is greatly pleased by Donkey's flattering and lets them pass.
Things often went roughly between Shrek and Donkey during the quest mainly due to Shrek's being reluctant about having Donkey as a forced "friend" and poor attitude, but by the end of the movie they were close friends.
After Shrek saved Fiona from marrying Lord Farquaad and Dragon swallowed the evil fiend, Donkey sang "I'm a Believer" at Shrek and Fiona's wedding reception and takes his love, Dragon, as his wife.
At the Shrek and the Swamp Gang Karaoke Dance Party, he sings "Baby Got Back" by Sir Mixalot. 
  Act I  "Overture" – Orchestra  "Big Bright Beautiful World" – Mama Ogre, Papa Ogre, Shrek  "Story of My Life" – Guard, Fairytale Creatures  "The Goodbye Song" – Shrek, Fairytale Creatures≠  "Don't Let Me Go" – Donkey  "I Know It's Today" – Young Fiona, Teen Fiona, Adult Fiona  "Welcome to Duloc" – Duloc Performers   "What's Up, Duloc?"
– Lord Farquaad, Duloc Performers  "Travel Song" – Donkey, Shrek  "Donkey Pot Pie" – Dragon, Donkey  "This is How a Dream Comes True" – Fiona, Shrek, Donkey, Dragon  "Who I'd Be" – Shrek, Fiona, Donkey  Act II  "Entr'acte" – Orchestra  "Morning Person" – Fiona, Bluebird, Pied Piper, Tapping Rats  "I Think I Got You Beat" – Fiona, Shrek  "The Ballad of Farquaad" – Lord Farquaad, Guards  "Make a Move" – Donkey, Three Blind Mice  "When Words Fail" – Shrek  "Morning Person" (Reprise) – Fiona  "Build a Wall" – Shrek  "Freak Flag" – Fairytale Creatures  "Big Bright Beautiful World" (Reprise) – Shrek  "This Is Our Story" – Fiona, Shrek, Donkey, Fairytale Creatures  "I'm a Believer" – Shrek, Fiona, Donkey, Entire Company≠ (as of October 2, 2009) Musical numbers.
Broadway.≠ Not included on the original Broadway cast recording.
"I'm a Believer" was recorded later and was released as a single, however. 
  Act I  "Overture" – Orchestra  "Big Bright Beautiful World" – Shrek, Mama Ogre, Papa Ogre, Fiona, King Harold, Queen Lillian, Villagers  "Story of My Life" – Guard, Fairytale Creatures  "The Goodbye Song" – Shrek, Fairytale Creatures  "Don't Let Me Go" – Donkey≠  "I Know It's Today" – Young Fiona, Teen Fiona, Adult Fiona  "Welcome to Duloc" – Duloc Performers   "What's Up, Duloc?"
– Lord Farquaad, Duloc Performers  "Travel Song" – Donkey, Shrek  "Forever" – Dragon, Donkey, Knights  "This is How a Dream Comes True" – Fiona, Shrek, Donkey, Dragon  "Who I'd Be" – Shrek, Fiona, Donkey  Act II  "Entr'acte" – Orchestra  "Morning Person" – Fiona, Bluebird, Pied Piper, Tapping Rats  "I Think I Got You Beat" – Fiona, Shrek  "The Ballad of Farquaad" – Lord Farquaad, Guards  "Make a Move" – Donkey, Three Blind Mice  "When Words Fail" – Shrek  "Morning Person" (Reprise) – Fiona  "Freak Flag" – Fairytale Creatures  "Big Bright Beautiful World" (Reprise) – Shrek  "This Is Our Story" – Fiona, Shrek, Donkey, Fairytale Creatures  "I'm a Believer" – Shrek, Fiona, Donkey, Dragon, Entire Company West End.≠ Cut before opening night. 
  Puss in Boots makes a silent cameo appearance during the "Travel Song".
He tips his hat to Shrek and Donkey and walks away.
Later, a bunny screams after Shrek calls him delicious.
A cow jumps over the moon, and a dish runs away with a spoon, while being pursued by the police.
Shrek and Donkey later see a giraffe and gazelles which look like the gazelle wheel in "The Lion King" (an excerpt from "Circle of Life" plays during this part of the song).
As with the 2001 film, Shrek ends the song "Travel Song" with "That'll do, Donkey, that'll do"; this is a reference to the similar phrase in the film "Babe", in which Hoggett reassures "That'll do, Pig.
That'll do."
King Harold and Queen Lillian originally sang a part during "Big Bright Beautiful World," explaining to Young Fiona that all ogres are monsters unloved by everyone.
Now they walk past Young Shrek, taking Young Fiona with them.
She waves at Shrek, but they lead her away (the guards preceding them give the impression that they are taking Fiona to her tower).
This was later cut out of the Broadway production.
A similar part was re-introduced for the national tour and London productions.
In "I Know It's Today" and "This is How A Dream Comes True," Fiona sings "Are you there, God, it's me, Fiona?"
which is similar to the book entitled "Are You There, God, it's Me, Margaret" by Judy Blume.
In "Morning Person," much of the part where Fiona dances with the mice is reminiscent of the nightclub scene in Bob Fosse's "Sweet Charity."
Most of the spoken lines are taken from the first film, including the Muffin Man scene between Farquaad and Gingy, and Fiona explaining the curse to Donkey.
"Welcome to Duloc" is the only original song from the film sung in the musical.
It is sung by the Duloc Dancers, who resemble the puppets that sing it in the film.
Originally the orchestra played "I'm a Believer" after the curtain call, but as of October 2, 2009 it is sung by the entire company at the end of the performance.
At the end of "What's up, Duloc?
", Lord Farquaad references Defying Gravity from the Broadway musical "Wicked", when he sings, "..and no ones gonna bring me down!"
followed by the famous ending note.
Also, the oft-repeated line 'Hey-nonny-nonny-nonny-no' is taken from the song "Shy" from "Once Upon a Mattress".
At the end of "Forever", the Dragon's final line is a reference to the "Dreamgirls" song "And I Am Telling You I'm Not Going".
Julie Andrews, who played Queen Lillian in "Shrek 2", "Shrek the Third", and "Shrek Forever After", provides the voice for the audio instructions before each performance that reminds the audience to turn off their cell phones, the use of recording devices is forbidden, etc.
and "if you refuse, a terrifying ogre will leap from the stage, lift you from your seat, and drag you far, far away."
During "Story of My Life", one of the show's musical numbers, Mama Bear sings "Mama's in the mud, Mama's in distress," which is a nod to the song "Rose's Turn" from the musical "".
During "Freak Flag" a flag is waved around with a face on it, which resembles the famous logo of young Cosette from "Les Misérables".
During a part of "Don't Let Me Go" on the national tour, Donkey sings to the music of "Take Me or Leave Me" and "I'll Cover You" from "Rent".
During "The Ballad of Farquaad," Farquaad makes several fairy tale references, implying that his mother was the princess from The Princess and the Pea and his father was the famous dwarf, Grumpy.
During "What's Up, Duloc?," the ensemble reference the musical "Chicago".  
"Story of My Life" is the first promo single from Smash Mouth's 2006 album "Summer Girl".
The "Story Of My Life" music video was filmed in an episode in Season 6 of the VH1 reality show "The Surreal Life", featuring all the housemates from that season, like Florence Henderson, Alexis Arquette, Tawny Kitaen, C.C.
DeVille and others.
The song has been used in promotion of "Shrek the Third".
This is third instance of a Smash Mouth song being used in some fashion for the Shrek film series.
"All Star" and a cover of the Monkees song "I'm a Believer" were both used in the first Shrek film.