askitter is a Haskell twitter client that uses the OAuth Twitter API. There's not much more to it. You can find a reference API at Twitter's development website.
Things that are present:
- Updating your status
- Getting your timeline
- Getting a user's public timeline
Things that are planned:
- Search
- Specifying that a tweet is in reply to a certain tweet
- List management
- Direct Messages
- Following and unfollowing people