Twitter-API-PHP is a PHP wrapper that is designed to make developing with the Twitter API as simple as possible. The wrapper is designed for v1.1 of the API and supports OAuth 2.0 and OAuth 1.0a.
Defensio, the anti-spam service, offers an API that can be integrated into blogs and web sites to fight spam comments and other unwanted spam (details at our Defension API profile). The service appears to be run by a small team, and is similar in purpose to Akismet, the much larger spam filtering service from Automattic (which powers WordPress.com), who also offer an anti-spam API.
Twitter has announced a brand new search architecture that indexes every public Tweet since 2006. It allows for queries that search through recent or popular Tweets.