With Twidroid, you can view the tweets of those you follow, post your own tweets, direct message, and reply. One of the coolest features is the ability to post photos. Well, kinda. Images are sent to phodroid.com and then linked in your tweet. Hint: attach the photo first, or the URL will over-write anything you've typed - same goes for the following paragraph.
There is some GPS functionality, but go to portrait mode too see all of your options on the non-scrolling menu (little bug there). They are: Insert Maplink, Insert Location Name, Insert Latitude/Longitude (seriously?), and Update Profile - for those who are just to lazy to type the full name of their city. Google Maps URLs are abbreviated via short.to, so none of the data should cramp your 140-character-limited micro-blogging style.
nanoTweeter, aTweeter, and a host of other dedicated programs may get the job done, but not as well as Twidroid. If you're looking for a more complete social networking solution, search the market for Twitter and you'll see which apps are compatible. BuddyMob comes to mind. But personally, I like dedicated apps. I don't want to carry any dead weight. Of course, what would go unused for me would be essential for others.
Twidroid's creators are called Zimmermann and Marban, which makes me think of an old-timey magic show. So I give it 5 stars.