OneRiot is a real-time search engine that searches both Twitter and Digg text and links. They crawl and index content a la Google.
There are 2 modes of search - Real-Time and Pulse. Pulse has the indexed content to add relevancy to the search. This is explained well by the TechCrunch piece when they launched in mid-May. Quantcast shows about 30k US visitors/mo.
OneRiot also compiles content, like hot topics, which can be added to iGoogle start page, Facebook, MySpace, etc. See their Apps section.
Thanks for posting, I’ll definitely be subscribing to your blog.