Well I got my Arduino Uno board a month or so ago and I've been dying to build up a quick project. I already got my Parallax Serial 16x2 Character LCD up and running, so I thought a simple add-on would be a basic Real-Time-Clock. The Dallas/Maxim DS1307 is a 5V I2C based RTC that looked pretty popular.
I soldered up the DS1307 to an Ardunio proto board and placed a CR2032 Coincell holder on the bottom. It fits nicely in between the boards when they are stacked.
A quick search on Google brought me to Maurice's software and a cut and paste later with a few LCD tweaks I had a working Arduino LCD Clock. Too simple; this literally took me less than 30mins to throw together.... I did this over a lunch hour and still had plenty of time to finish my sandwich :-)