Lack of Twitter support was one of the main hangups I had with Windows Phone 7 at launch, but we quickly saw 3rd party applications launched for this functionality. However, it wasn't until now when one that was truly useful appeared with both live tile and toast (top of the screen notifications) support. Beezz just rolled out on the Marketplace with this notification support, version 1.1, and is available for FREE.
The first Twitter app launched on Windows Phone 7 was Seesmic, but it didn't support the native keyboard functionality so was uninstalled right away. Keyboard support has been added, but there are too many screens for me so I do not use it. The official Twitter app was next and it works fairly well. However, with no notification support it is a real pain to have to go into the application and wait for it to refresh to see your latest Twitter stream.
You can find Beezz in the Marketplace (link to the Zune desktop) and give it a try. It has now earned a spot above the fold on my Start screen. There is no multiple account support, no ability to upload videos (may be an OS limitation), and no conversation view. The lack of conversation view kills me and is what I hope they add as a priority for the next update.