ResearchBuzz has an interesting post about Yahoo's lack of a limit on query strings for searches. Tara has one example with a search restricted to only state government sites (i.e. a search restricted to matches on one of the 50 domains wa.gov OR ca.gov OR ...).
By exploiting this, someone could build a bunch of interesting little tools. For example, a tool could restrict a search to only the blogs you read, the sites you have bookmarked, or even (using a toolbar) the sites you viewed in the last N days. Tools could do various forms of query term expansion (e.g. turning a search for "greg" into a search for "greg or gregory"). At the extreme, if you're willing to seriously abuse the system, you might even be able to use Yahoo search to find related pages or clusters of web documents on the fly by searching for subsets of keywords or phases in common.
Of course, I wouldn't count on Yahoo maintaining this feature. These kinds of very long queries have to be expensive on their servers. Tara's state government search demo took 5-10 seconds to respond when I tried it. But, if you have the time, it might be fun to play with it while it lasts.