The 140dev Twitter Framework is a free source code library containing a Twitter database server for gathering tweets for selected keywords, as well as a display plugin that lets you show tweets from the database in a single line of PHP code.
Hellotxt, the service that lets you simultaneously update your status and read your friends' status across many services, has now released an API. There is a long list of microblogging and social networking sites that Hellotxt interacts with, ranging from Facebook and LinkedIn to of course Twitter and FriendFeed (see their home page below).
Yammer, a microblogging-for-business site and winner of the top prize at the recent TechCrunch50 conference, has released an API to allow developers to build their own applications around the service (Yammer API profile). Created by the people behind genealogy site Geni, Yammer has taken the tried and tested Twitter model, which asks users to answer the question "What are you doing?" in 140 characters or less, and adapted it for businesses, turning the question into "What are you working on?"