As of now, I'm sure will change as time goes on, in no particular order...
1. Where - I use this almost daily. Has locations of just about anything near you, provides directions, numbers, menus, sometimes hours and you'd be surprised at how many user reviews there are. Also lists local events and all kinds of stuff. Have had it since my Eris. Extremely fast, voice search options and links directly to navigation. Love it. I couldn't be without it.
2. Handcent - I've tired stock, GOsms, and one other one (forget what it was), and Handcent is easily the best. Gosms wasn't bad, but I think handcent is more customized.
3. Google Music - Uploads all your music to a cloud which can be streamed from anywhere at anytime. Not sure if it uses data, but I'm on unlimited so meh. Was using before Zumocast so I haven't tried it yet.
4. Netflix - Been using this all the time since it finally came out and was reliable.
5. Dark Galaxy and NESoid emulator - Two separate games but I had to include both.
I'm sure this will change monthly, but these are some of the first things I put on my Bionic. And for good reason.