Let's gather a list of current options (feel free to edit this answer):
- [whodidit][1] has a nice interface, but might miss changesets if they only touch relations for example.
- [osm filter][2] (see Malenki's answer) uses heuristics to weed out uninteresting changesets, but the heuristics have false-positives.
- [ITO world][3] is usefull too, but it requires a login and can be a bit hard to navigate for our purpose.
- osm.org's [history tab][4] suffers from worldwide edits that get included when they shouldn't.
- [OWL][5] (see Richard's answer) is the traditional tool, currently offline but should come back.
- The [new history tab][6] 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][7] feed shows new contributors, who often make a few mistakes when they start.
- [osm qa feeds][9] 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][8].
[1]: http://zverik.osm.rambler.ru/whodidit/
[2]: http://positron96.appspot.com/osmfilter.html
[3]: http://www.itoworld.com/product/osm/map
[4]: http://www.openstreetmap.org/browse/changesets?bbox=-0.11626%2C51.50403%2C-0.0993%2C51.51077
[5]: http://matt.dev.openstreetmap.org/owl_viewer/
[6]: http://owl.apis.dev.openstreetmap.org/
[7]: http://resultmaps.neis-one.org/newestosmcreatefeed.php
[8]: https://wiki.openstreetmap.org/wiki/Change_monitoring#Monitoring_Toolshttps://wiki.openstreetmap.org/wiki/Change_monitoring#Monitoring_Tools
[9]: http://tyrasd.github.io/osm-qa-feeds/
