Firefox

Great open-source browser, now my default. Worth it alone for its Adblock extension, though otherwise satisfactory.

One weird bug on the Mac version that's been around for a while: if you have the Downloads window open downloading a file and close your main browser window, the menus and most key commands become inaccessible. You can, however, switch away from the program by the usual methods. Quick workaround: find a linked URL in another application (like Mail) and click on it -- Firefox will open a new browser window for it automatically, and you'll be good to go again. UPDATE: It appears that in the new version 1.5 Beta 1 this has been fixed. Thank goodness.

Quick little timesaver tip: Command-1 through Command-9 switch between open tabs in a given window.