Today, I added four sites to the Google Co-Op search engine.
Two are tax specific, Tax Playa and Andrew Mitchell International Tax Services. (The Andrew Mitchell site has the incredible collection of tax flow charts that Jim Maule, justifiably, raves about.)
The other two, Docuticker and Open CRS Network, have a large amount of research material. While the majority of the material on those sites is not tax or business related, the quality of what can be found there makes it almost imperative that these sites be included in the search engine.