MagiCal is a cool little calendar application from Charcoal Design. As you can see it just puts a drop-down calendar in your menubar for easy access. In addition, you can actually "tear off" a month and leave it on your desktop (handy for making reservations, I find). MagiCal can also add the date and time in a customized format to your menubar, if you want to use it instead of OS X's built-in clock.
While my favorite menubar calendar remains the more full-featured MenuCalendarClock, it is $20 and MagiCal is a free download (donations requested). One last cool thing about MagiCal is that the developer offers three versions for download: Universal Binary, PPC, and Intel. The single platform ones are indeed about half the size of the UB. Personally, I wish more developers would consider doing this.