Yahoo Search Clues is an interesting service, somewhat reminiscent of Google Trends.
Punch in a keyword and it will show you traffic over time. You can also compare two keywords.
It shows the demographics of who is searching for the term, by age, gender, income level, and geography.
It also shows popular searches before and after this search. This can be helpful in figuring out what searchers are really looking for. Finally, it also displays related keywords.
This is another useful tool for your keyword research toolbox.