I just finished (are you ever really finished with a project) an online accounting system for my old dormitory, where we would essentially buy cheap beer in Germany and sell it to ourselves.
We needed an accounting to keep track of who drank what, how much, and what they owed the common purse.
I really want to make an online cookbook with user added recipes, so that might be my next project, if I ever get the time.
Out of Rails scope: I am currently hacking together the songbook for the Computer Science study line at Copenhagen University, where we have approx. 128 songs in our songbook each year.
This is made with LaTeX (of course) and then some Perl, Python and Bash script to glue it all together.
I am currently rewriting our flow, so instead of one long page with lots of options the user has to think about, it will be four smaller pages, each with little interaction (only a couple of mouse/key presses per page)