A great 4 min video about optical illusions.
Listening to MP3 audio books in iTunes and your iPod is troublesome. They show up as regular songs and not in the special "Audio Book" category, thus options like changing speed or remembering playback position are not available.
You can find many tutorials about converting MP3s to Audiobooks. Most of them tell you to join multiple MP3s to one file, convert this file to .AAC in iTunes, rename its file extension and readd it to the iTunes Library. That might work well, but you might also use the free and Open Source tool Audiobook Maker.
Sling is a addicting flash game. Your job is to steer a small green ball of glue through fifty increasingly difficult levels.
The OS X Dashboard (and all similar implementations for Windows, Unix, ...) is great for little tasks like looking up a word in a dictionary, or doing simple calculations with a calculator.
Regrettably the calculator widget shipped with OS X 10.4 is really bad. It doesn't honor the regular order of operations, thus entering
1+1*2 leads to
4 instead of
3, and it's very limited in functionality.