Assuming the Amazon Kindle uses MobiPocket Reader (or at least completely supports .MOBI files), it supports many more formats via "desktop" (Windows XP) conversion: .doc, .rtf, .pdf, .html, .chm, (.ocf) and .txt. I don't know if MobiPocket Reader supports .txt without conversion.
Also, .PDB is Palm Database File format but this covers many incompatible file types. What MobiPocket supports is AportisDoc, which is the main Palm e-book format. Other .pbd file types include zTXT, PeanutPress, TomeRaider, Plucker, and iSilo. There is a brief discussion of this on the FBReader Google group
here.