Elf

Elf is a very cool and useful idea: enter the details of your public library, and it'll notify you by email and by a personalised RSS feed when your books are due to be returned, and when holds have arrived. I currently owe something like $35 to the ACT library because I am seemingly incapable of returning books on time: hopefully this will save me some money! Luckily the two libraries I frequent - the ACT and Tasmanian public libraries - are two of the four Australian libraries available on the site.

I found out about this website via Lifehacker - if you don't read that website daily, you should start immediately. I have learnt so much truly useful stuff from that site...