Some times I use a tool for a while before it appears in this list.
I stumbled upon, StumbleUpon a while ago, while tracking the visitors to my blog. It is one of those compelling applications that you have to have. It is sitting in my Firefox Toolbar, right now. StumbleUpon made news this week. They were acquired by eBay. They seemed to have added more services too. Definitely worth checking out.
Swicki is a cross between wikis and search engine (not sure why they decided to spell wiki as wicki). The swickibuilder lets you build a search (a few terms and information sources) and use it collaboratively. According to the swicki FAQ:
A swicki is new kind of search engine that allows anyone to create deep, focused searches on topics you care about. Unlike other search engines, you and your community have total control over the results and it uses the wisdom of crowds to improve search results. This search engine, or swicki, can be published on your site. Your swicki presents search results that you’re interested in, pulls in new relevant information as it is indexed, and organizes everything for you in a neat little customizable widget you can put on your web site or blog, complete with its very own buzz cloud that constantly updates to show you what are hot search terms in your community.