I've been using Overture, WordTracker , Keyword Discovery and Keyword Country for quite a while.
Overture's results tend to be highly skewed due to automated searches. Combine that with the lumping of singular and plurals and the incorrect ordering of many phrases and it loses it's usefulness.
WordTracker has a fairly small db, but it can give you some fairly good data until you get to very small niche phrases at which time it loses it's usefulness.