How can I set up automatic notification whenever data that I've added to OSM is changed?

Let's gather a list of current options (feel free to edit this answer):

• whodidit has a nice interface, but might miss changesets if they only touch relations for example.
• osm filter (see Malenki's answer) uses heuristics to weed out uninteresting changesets, but the heuristics have false-positives.
• ITO world is usefull too, but it requires a login and can be a bit hard to navigate for our purpose.
• osm.org's history tab suffers from worldwide edits that get included when they shouldn't.
• OWL (see Richard's answer) is the traditional tool, currently offline but should come back.
• The new history tab doesn't yet do notification, but its backend should allow for that, and it is slated to become the new official change-notification service.
• The newest active osm feed shows new contributors, who often make a few mistakes when they start.
• osm qa feeds helps create rss feeds for many tools

All those tools work by letting you subscribe to a RSS feed. The wiki also has a list of monitoring tools.

Here you can drop the link of the osm.org history page of the area you want to monitor. The result is a RSS-feed without the big changesets bots and some mappers cause.

hth
malenki