Remember how disappointed I was to find out that Mastheads.org had switched to subscription access? Well, I've now found a useful free "substitute." It may not be quite so complete, but you can help change that (as I'm trying to!).
The AbsoluteWrite.com discussion board "Current Magazine Mastheads" section contains a number of magazine mastheads, updated and augmented as we practicing writers update and augment them (I've tried to help out with that a bit this week myself). Check it out and contribute some information, too!