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This twitter app handles multiple columns for easily following your friends, mentions and direct messages. It recently gained the ability to work with lists and the columns can be dragged and dropped in the desired order. This is one of the few twitter apps I have used on any platform that works just as well in portrait and landscape orientations. Spaz HD is a great twitter app and is open source and produced by volunteers. Free.
Google Reader users that have a lot of RSS feed subscriptions may want the fastest app for keeping up with them. While NomNomNom is a great attractive app, TouchFeeds is optimized for processing a lot of feeds and articles as quickly as possible. This is the app I go to first for processing my Google Reader feeds. $2.99.
TWiT is the best online video network bar none, and this app makes keeping up with the great shows a breeze. It is similar to the Rev3 app mentioned above, and like that one turns watching TWiT video into a thing of beauty. Free.