All I use my phone for is phone calls (obviously), Google Maps (when I'm lost in Boston), and SelectRadio, which I use to stream Sirius on my 15 min walk to my office.
SelectRadio is not free ($25), but it's easily one of the best streaming apps I have ever used. It connects to online radio stations as well as internet radio services such as Sirius and XM. I love it.
Currently there aren't any good Droid apps for Sirius. There is a free one, but they restrict some of the content. I have Pocket Tunes on my iPod Touch (another fantastic app). Since my next phone will probably be a Droid, I really hope someone will develop one. Then again, I may get an iPhone (LTE) if it launches on Verizon in the spring. Would be an easy transition since I already have tons of apps for my iPod Touch.
I wish my phone was more powerful. It's very underpowered and very slow. Also, it's not 3G, and it's Windows Mobile 6.1. I really need a new phone.