I have just had it a few days, but here is what I have done so far. All of this stuff is free on the market....
Google Stuff:
Google voice
Google Latititude
Google Goggles
Google Shopper - can't get this to work yet...camera error
Google Sky Map
Other Stuff:
Mint.com
Pandora
Vuvuzela (moonbeam's version)
Shazam
IMDb
YouVersion Holy Bible <-- excellent
3G Watchdog
I highly recommend the 3G watchdog if you are on a tiered data plan. It helps you keep track of what your data usage has been. You can put a widget up on your screen that calculates it in real time.
Updates
7/14/10
Setting Profiles Lite - Allows you to change settings (sounds, wifi, etc.) based on different variables (time of day or location or time and location). Testing out to see which is better..
Timeriffic - Same as Setting Profiles Lite, but maybe has full capabilities (instead of the Setting Profiles being the "Lite" version? I'll report back.
Google Translate - probably handy to have from time to time.
Epocrates - Haven't set it up yet...not sure if it will be useful