I recently downloaded and installed MMPlayer on my Palm and I like it so much I had to share!
MMPlayer serves as an MP3 player for me, but it can do so much more. It is a media player for the Palm OS5 and it's capable of playing a number of standard audio and video files. Movies can be compressed, stored on a memory card as an avi or mpeg, and watched using MMPlayer. It can also play streaming media directly from a server or on the Internet if your Palm has a usable network connection. It's completely configurable and skinable too!
MMPlayer can also read .m3u playlists, display your free memory, and bookmark your place while watching a video so you can come back to it later. The program can collect statistics based on how hard the file you've chosen was to play so you will know if you need to encode it again at lower settings. Also, the program provides benchmark tools that can be used to see how fast the program can play a file on the current device.
The program isn't free, but it's cheap (less than $15) and it's pretty easy and painless to install the key once you have registered. Their home page offers an online manual, FAQ, contact information and forum as well as a collection of skins (and instructions on how to design your own skins if you like).
Now all I need is a 1gb CF card to put my tunes on and I'll be set!