I have a PPC-6700 and the software I use on it is:

Pocket Controller
Great for Remote viewing/controlling of your phone over your active sync connection

PDAnet
Alows me to use my Unlimited Data connection on my phone as a modem for my PC

WisBar Advanced
Alows you to theme your today screen.

SPB Diary, Pocket Plus, Finance

psShutXP
Allows you to shut off the screen, soft reset etc

VJCandela
Allows you to set a button to turn on and off your camera flash, so you can use it like a flash light.

Voice Command

iGuidance
when paired with a BT GPS Reciever this is pretty nice to have since it does reroutes compaired to google maps and Live.. granted it costs compaired to those but still nice.

and last but not least, I am running a Clean & Simple AKU 3.5 ROM from one of the great guys at PPCGeeks.com They are in the process of making a completely stable 3.5 right now.. They got alot of information on how to build your own rom so you can include stuff in the OS it self to free up your memory for more applications. Very good site to read if you have a PPC.. Also PDAPhoneHome is another one.. between the 2 if you run into an issue on the WM phones someone should be able to help you figure out whats wrong and maybe fix it. Hope this helps a little..