You can do pretty much everything from Seesmic that you can do directly from twitter, plus more. Want to post a tweet you saw to Facebook, text or email? No problem. Upload photos and video from your Android are easy too. Downside of Seesmic? It can't handle multiple twitter accounts and it takes a few taps to get the Compose screen. But for now, it is my twitter App of choice.
Update: Seesmic has added support for multiple accounts in a new update. Now I love Seesmic unconditionally!
Phones tested on: Nexus One