Firefox/Chrome/Opera/IE: I've never found Google Calendar to be particularly unattractive—until I compared it to the sleek, clean look of Google Calendar with the Helvetical user script, that is.
Helvetical has a very similar design aesthetic to previously mentioned Helvetireader, a Helvetica-themed Google Reader customisation. It's easy on the eyes and drops out a lot of the clutter of Google Calendar while keeping all of its functionality in tact.
Helvetical works with any browser that supports custom user scripts, which—in theory, at least—includes Firefox with Greasemonkey, Chrome with user scripts enabled, Opera, and IE with IE7Pro. (We've only tested it in Firefox.) If you give it a go, let's hear how you like it in the comments.
Helvetical [I am Ad Taylor]