I learned about tsort today. It's a UNIX utility that takes as input partial orderings and returns sorted output. This can be used to determine if a directed graph has cycles since it prints to STDERR if it finds one.
It would be nice if /dev/null/* could be written to. For example echo "foo" > /dev/null/foo gives an error, but really, why should it? (In that same discussion the facetious idea for subdirectories of /dev/random came up. For example /dev/random/even, /dev/random/prime, and /dev/random/gaussian.)
I recently found a new use for my digital camera. I was hiking in a park in Portland and needed a map. There were no paper maps available, just one big plywood one. One snap later and I had a great 4MP map of the park ready for me to zoom in and scroll around on.