I think
BackupMan works very well. Also,
BackupBuddy,
RescoBackup,
BackupPro,
RescoExplorer (a file manager with backup features) are some popular ones. I think they all support backup to the SD card, which is extremely important should you encounter problems. Simply run the backup app from the card and restore the files and you're back up.
I've mentioned backup solutions in
this posting on my Palm blog.
Freeware alternatives include:
BackupLite,
UniCMD (a file manager that has a backup utility),
InnerBackup,
CardBackup,
VfsBackup,
PowerGuardLite,
RedFelineBackup,
SmartBackup.