FontSmoother lets you replace the Palm fonts (on hi-res OS 5 devices) in almost all applications with nice, smooth fonts. Lots of fonts are included, and you can also create your own fonts from Windows TrueType fonts (assuming the ttf font license permits this).
Version 2.20 improves highlighting on Treos (a big thank you to testers!) and allows fonts to be stored on a card. As always, upgrades are free: just turn off the old version, and install the new trial on top of it.
Download from:
http://zlthemes.com/Programs.php
http://mobihand.com/product.asp?id=5849&n=FontSmoother
(I've submitted updates to other sites, but not all of them have gone through yet.)