Dopplr is a great social-networking based tool for keeping a track of your travel plans.
The idea is that you input your travel details and it will find other people going to the same location (or nearby) at the same time.
If your social network members keep their details up-to-date then you might just find yourself engaged in an impromptu party in a foreign city instead of watching TV in a language you don't understand whilst slowly emptying the mini-bar and your wallet.
Another feature of Dopplr, if you're interested, is the ability to calculate your carbon footprint.
Dopplr even provides webcal links so that you can subscribe to your travel calendar. Using this functionality, Google Calendar and BusySync, the details on from my Dopplr account appear on my iPhone, automagically. Handy!
Another weapon in my online arsenal of useless rubbish. ;)