I've found a few lists of sites for finding niche ideas, I filtered out some of them and compiled the result here, so that I can easily find them again, it's better than a bookmark :-)

Google Insights for Search http://google.com/insights/search/
Google Trends http://www.google.com/trends/hottrends
WordTracker GTrends http://freekeywords.wordtracker.com/gtrends/
Google News - Most Popular http://news.google.com/news?ned=us&topic=po

SEOmoz - Popular Searches http://www.seomoz.org/popular-searches

Yahoo! Buzz Index http://buzz.yahoo.com
Lycos 50 http://50.lycos.com