Be very careful with this thing called "traffic" to valuate domains, and to buy domains. Traffic can be easily manipulated with scripts and proxy networks. Don't depend on webstats, but other external benchmarks to help indicate that the name is built, at the very least, on solid ground.
Nonetheless, Overture is a good starting tool.
My favorite dipstick? I find that Google Adwords can give many clues as to the value and popularity of different keywords given different circumstances.