I've been meaning to mention my other linux operating system, Puppy Linux. It's tiny, just 150MB total. It can be booted from a flash drive and run completely in RAM if necessary, no hard drive needed. And it runs very light—I've using it on my 12-year-old Thinkpad X24 (with just 256M RAM). This laptop would be dead without Puppy.
There are lots of varieties of Puppy Linux. I've heavily edited the one I use, so it's pretty unique. I've really enjoyed learning about how computers operate and seeing how well I can make it run and how nice I can make it look:
These are thumbnails, click to view larger. The theme is Owl Light. And the icon set is called Faenza.