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To the developer of a GM sound bank, one of the biggest problems is testing out each of your voices. You need to play a MIDI file that not only plays each voice, but you have to know which one is playing. In addition, the instrument has to be played correctly, with the proper controller, or the "synthy" sound you think you're hearing may not be the fault of your voice, but how the voice is played. There's a few sites I've found to help, but the best is: Buzzwood MIDI test pages.

bulletWarren Trachtman has a great piano soundfont on his page, as well as some great ragtime piano music. The piano is such an integral part of music you need a realistic one, and here it is.
bulletJohn Sankey has a great harpsichord soundfont (it's on on this site - german8.zip) but this is his home page. Lot's of interesting information about his musical background. You'll never think about a harpsichord the same way again after visiting there.
bulletFrank Wen is developing the Fluid soundfont - a 48MB Monster that could be the ultimate. Check his page for progress updates on this massive undertaking.
bulletDejan Kamenšek has made the best Distortion Guitar soundfont I've heard - I use it and you should too. You can find it on this page.