Having blogged about the Ralan.com site yesterday, I need to give equal time to Duotrope Digest. I keep a standing link to this one in the list at left. It's a bit different because it relies on folks to submit info on their on response times for specific markets, and it even offers a tracking service, so that you can record your submissions as you send them; when you hear back, you enter the response date. It calculates how long it took for you, and it adds your numbers and the response (format and whether it was a rejection, acceptance or rewrite request) in with other folks' responses to generate statistics for that market. It allows you to get a list of only your own submissions so you can track them online.
It's pretty cool, also free, and they're in desperate need of donations, so check them out!