I have two...
1st. SharkModem is sweet, but it would be twice as sweet if it could turn into a real modem as opposed to a proxy, ie. it's identical to connecting to a dial-up connection. I can't get into my MySpace on my laptop and I go through withdrawls!
2nd. a GPS program that takes advantage of the Location Based info on a 7xxx series blackberry (ie. my 7250...)