palm
software
Quick News
This is a pretty slick RSS reader for Palm devices. It supports images, podcasts, live updates for web enabled devices, and will help you search for feeds. Shareware, but well worth the $15.
Keyring for Palm OS
Keyring is for storing passwords and other private info - from website passwords to banking info or whatever. It has a master password, and will generate passwords for you as well. I like KeePass for on my desktop... but until they make a Palm version, Keyring is the next best thing.
MiniTones
MiniTones reads MP3 files off your expansion card on your Treo or Centro and lets you add them ass a ringtone. The main downside is that it doesn't convert to midi, so it can use up your internal memory pretty quick. You can split the file down to pull just a section of the song though.
Palm Freeware
Site with a LOT of freeware stuff for PalmOS

I have owned a PDA since the Geos Zoomer, and Palm based ones since the Handspring Visor came out. I currently use a Centros smartphone, and love it. It has a TON of built in software, but here's a few things I've added to mine... as well as links to different resources as I find worth sharing.