Probably the most popular is the '59 Fender Bassman amp, which is shown in the picture. These include the old Gibson and Silvertone tube amps. New tube amps are also being made, including the Crate VC series. The VC20 is shown above, but currently many harp players have been praising the VC508. These are lower power amps, which are also smaller and lighter in weight.
Many players prefer the sound of the small tube amps.
They work particularly well in small clubs where loud volume is not a concern, but they also can be mic'ed through the PA to amplify the volume of that sound and put the harp in the monitors. One of the most popular small tube amps is the old tube Fender Champ. There are many many solid state amps out there, some of them quite inexpensive, and even though they are not preferred for the classic Chicago Blues sound, they have their own character and can be used for amplified harp, depending on your taste, desires, and needs. The Pignose amps also work fairly well for distorted amplified harp, and they are very portable. The Hog shown above delivers about 10 watts, which is pretty loud for its size, and runs on a large rechargeable battery.