There are three programs/utilities I think are essential: (Listed in order of importance - at least to me!)
1. Ilium Software's eWallet (For obvious reasons)
Ilium Software
2. Resco's Resco Explorer 2003 (v5) - a must have if you are moving files around.
Resco.net
3. Sprite Backup v2 - by far the most powerful, robust, backup option out there ...
Sprite Software.com
There's many more I could list, but these are the three "must haves" and are always the first ones I install on a new PPC... Have fun!