Here's a great hack for my mac-user friends to add the date to the menu bar:
You go into the International option in your system preferences: a place I would never have thought to look. Simple and easy with a few options for modification.
I've recently downloaded MagiCal (http://www.charcoaldesign.co.uk/magical) which offers an alternative to the system date/time option. It also gives you a full monthly calendar with one click on the menu bar. I had a calendar widget on my dashboard but it seems like it always took so long for the widget images to open up and appear on my desktop that it wasn't really worth it. MagiCal lets you pull the calendar off the menu bar so you can move it around and have several months open at a time. There are a lot of options you can select to really modify and customize it to your taste.