Admittedly, I'm based, but here goes (in no particular order):
- Plucker (offline HTML and ebook reader, lots of functionalty, beats the proprietary compeition in size, speed, platform support, and features)
- Vindigo
- BackupBuddy "Network Edition" v4.1 (old, but the only one that still handles backing up to ftp)
- AeroPlayer (ogg-only)
- FileZ (File manager)
- Converter (unit, distance, etc. conversion)
- KsDatebook (incredible stock Datebook replacement)
- MyCheckbook
- Hours (client, billing and timetracking)
- IPCalc (Alan is good people)
- GNU/Keyring (bulletproof password/data security vault)
- PgmrCalc (Programmer's calculator, a must!)
- Mergic Ping (basic ping application, no frills)
- Preference Editor (Removes "crumbs" left by installed and uninstalled apps)
- Progect (GPL Project Management tool, beats the non-free alternatives in features and power)
- Qlaunch (map apps to buttons and various extended presses)
- SNTP (Simple Network Time Protocol for the Palm)
- Timesheet (a slightly different billing and timetracking application, I use this in combination with Hours above)
- TealScript (I abhor Graffiti2, and TealScript seamlessly gives me back Graffiti 1)
- UnDupe (remove duplicate records from your core Palm databases, a lifesaver when testing new sync code between applications)
- Palm WebPro Browser (standalone web browser with Javascript, cookie, and cache support - works well over Bluetooth on my T2)
- xTIME (clock, stopwatch, calculator, alarm, etc.)
- ZLauncher (replacement application launcher)
- Strip (Secure Tool for Recalling Important Passwords)
- OmniRemote Pro (IrDA app to replace television and stereo remote controls, great app!)
Those are the top 24 on my Tungsten T2 that I run on a regular basis. I have ~200 applications installed, but those are my "daily-use" ones.