Simon Budig's woof is my current favorite application for dead-simple file sharing.
So you want to share a file with someone, but you don't want to email it, IM file transfers are a bit sketchy and neither of you want to create a temporary ssh/ftp/whatever account somewhere just to transfer one single file?
$ woof hamster_dance.mp3 Now serving on http://188.8.131.52:8080/
Woof serves it over HTTP in a second. It's a small python scripts that fires up a web server, redirects all requests to the file you're serving, and shuts down after serving it the number of times you want (once, by default).
If you point it to a directory, it'll automatically serve a .tar.gz of it.
(My only gripe is that it can't do a directory listing instead. Sometimes I want to share a directory and say "take what you need" rather than "here's every bit of it, tarred and gzipped".)
It's super-sweet. Try it. You'll like it.