Answers:

1) http://www.toughestdeveloperpuzzleever.com/tdpe/level1.aspx

Click on the knob

2) http://www.toughestdeveloperpuzzleever.com/tdpe/level2.aspx

Read sentence and type this:

3) http://www.toughestdeveloperpuzzleever.com/tdpe/level3.aspx

View page source and type this:

4) http://www.jeffblankenburg.com/tdpe/jeff.aspx

View source, copy-paste to calc in bin mode, convert to hex, pair of 2 hex:
U: 69,68,65,61,72,74 and P: 73,6F,66,74,77,61,72,65
open an ASCII hex chart table and translate (61-79 is a-z lowercase):

Type Username: iheart Password: software and Proceed

5) http://www.jeffblankenburg.com/tdpe/thatisnotenglish.aspx

View source, image is called wingdings.png, open WordPad, type a-z in lowercase in wingding font
match the image symbols, convert to arial, copy-paste.

Type Username: wheelof Password: jeopardy and Proceed

6) http://www.jeffblankenburg.com/tdpe/yesthatisjavascript.aspx #cns

Read the script, copy/paste in an HTML dummy page, fix bugs, it should be:

regexp.exec() (c missing)
checkNonInteger() (n missing)
primes[] (s missing)

Type solution: cns and Proceed

7) http://www.jeffblankenburg.com/tdpe/algebra.aspx

Substitute downward until you get:

1+x+x+x+x=89
x=(89-1)/4

Type x=22 and Proceed

8) http://www.jeffblankenburg.com/tdpe/sudoku.aspx

By simple deduction, it cannot be 2,3,5,6,7 so you are left with 1,4,8,9
Either solve the puzzle or just try them all four, one by one.

Type x=9 and Proceed.

9) http://www.jeffblankenburg.com/tdpe/BOSSLEVEL.aspx

Two different ways:
Open the JavaScript code translate into keys and do it.
Search on Google: "Contra 30 lives"

Open IE for this one, it is broken in Firefox 3.

Press: UP,UP,DOWN,DOWN,LEFT,RIGHT,LEFT,RIGHT,B,A

10) http://www.jeffblankenburg.com/tdpe/notourprincess.aspx

View source, click the image...

11) http://codinggeekette.com/tdpe/mybff.aspx

View source, look for that sentence in Google.

Type: http://www.codinggeekette.com/tdpe/mybff.aspx?user=software&...

12) http://www.codinggeekette.com/tdpe/monumental.aspx

Does not work anymore...
Type: http://www.codinggeekette.com/tdpe/sequencename.aspx?rnd=fib...

13) http://www.codinggeekette.com/tdpe/calcwords.aspx

http://en.wikipedia.org/wiki/Microsoft_Bob (Hint: Melinda Gates was the project manager)

gOOgLE, BOB, BIOS, LEgOS

(reverse with digits, if not convince try with a real calculator)

Type: 376006, 808, 5018, 50637 and Proceed

14) http://www.codinggeekette.com/tdpe/notasnake.aspx

Search on Google: "Zen of python"

Type: import this and Proceed

15) http://www.codinggeekette.com/tdpe/idclipdoom.aspx

Search on Google: "Doom I" cheat code next level

Type: IDSPISPOPD

16) http://www.codinggeekette.com/tdpe/showmethevariables.aspx

Try the common server config display
http://php.net/phpinfo

Type: phpinfo();

17) http://www.codinggeekette.com/tdpe/oldschool.aspx

http://en.wikipedia.org/wiki/ENIAC, wikipedia says 1946... try 1945.

Type Username: eniac Password: 1945 and Proceed

18) http://www.codinggeekette.com/tdpe/chinchillas.aspx

Search on Google: smalltalk chinchillas implementation

http://www.squeak.org/

Type: squeak and Proceed

19) http://www.codinggeekette.com/tdpe/icanhazcode.aspx

Search on Google: "CAN HAS STDIO?"

Example ends with: KTHXBYE

Type: KTHXBYE and Proceed

20) http://www.codinggeekette.com/tdpe/kthxbye.aspx

View source, click the image

21) http://brendan.enrick.com/tdpe/untitled.aspx

Use code above to find the correct number.

Type: 1162564096 and Proceed

22) http://brendan.enrick.com/tdpe/icecream.aspx

Call ancestor class first in order (implicit super() being called)

Type: 134124 and Proceed, then click on the link

23) http://brendan.enrick.com/tdpe/threelawsofrobotics.aspx?x=Ro...

Search on Google: "robot laws"
Answer also in view source

Type: http://brendan.enrick.com/tdpe/threelawsofrobotics.aspx?x=As...

24) http://brendan.enrick.com/tdpe/level24.aspx?x=superSecretExt...

Open level24.js or secret.js

Type: thisistheanswer and Proceed, then type URL:

25) http://toughestdeveloperpuzzleever.com/tdpe/level25.aspx

Read 404 message, compare to a real one, it says in suffix:

", but it's more likely you're being fooled."

View source, read stack trace:

"at System.Web.UI.ThisIsAFakeErrorPage.YouCanGetPastIt.GetToNextPuzzle(ToughestDeveloper puzzleEver, DotCom slash, TdpeSlash level, TwentySix dotAspx)"

26) http://toughestdeveloperpuzzleever.com/Tdpe/levelTwentySix.a...

View source, either use show cookie, Firebug or HTTP Live headers:

Cookie header says: nextLevel=--> http://www.toughestdeveloperpuzzleever.com/tdpe/twoseven.asp... <--;

27) http://www.toughestdeveloperpuzzleever.com/tdpe/twoseven.asp...
and http://jexed.com/tdpe/twentyseventext.aspx

"Take the first one each time. They'll get you the distance."

The author means take each page name and plug it into the leveinstein function,
you will get the word distance of 8, which is the number of words in:
"This is not the hint. Seriously stop checking."

Search on Google: distance algorithm

http://php.net/manual/en/function.levenshtein.php

28) http://jexed.com/tdpe/levenshtein.aspx

<?php echo levenshtein("killer","bunnies"); // output 6

Type 6 and Proceed

29) http://jexed.com/tdpe/pingpong.aspx

Now, compare those two images:
http://jexed.com/tdpe/assets/images/pingpong.png

Search on Google Images: ping pong tournament
http://farm2.static.flickr.com/1121/1098453615_f41fb895bd.jp...

The title is different, now read the sentence:
"There's a command in that image. Maybe you should start following instructions."

open a command prompt, type: "ping pong.com",
you get: "Pinging pong.com [68.178.232.99] with 32 bytes of data:"

Type one number in each box "68 178 232 99" and Proceed

30) http://jexed.com/tdpe/congratulations.aspx

View page source and figure it out yourself :D