Heimdall is an antigrief plugin for Bukkit. You can read more about it on the dev.bukkit.org project site:
http://dev.bukkit.org/bukkit-plugins/heimdall/
While the original intent was to have a plugin that could keep an eye on the server and autoban griefers, it only made it halfway to that goal since it turned out my mods enjoyed the duties of keeping an eye on griefers. So at this time the Heimdall implementation is primarily a griefer notification and logging engine; it can detect griefing activity on your server and notify your mods as well as log the activity for easy review later.
However, I no longer consider myself an active Bukkit developer. I occasionally have some free time and some motivation to go with to do small enhancements, but Heimdall was an ambitious project from the start so it seems unlikely I'll ever get back to finishing off the long list of ideas I had for it.
The good news is that it is stable and works well in it's current form and keeping it in a working state shouldn't require a lot of effort thanks to the relative stability of the Bukkit API. So for admins who are looking for what Heimdall has to offer today and who don't mind the rough edges of very sparse documentation, it can be a great tool to add to your arsenal of antigrief plugins.
Should you wish to contribute to the project, you can look over the TODO list for some ideas or certainly feel free to come up with your own. Also please see the CONTRIBUTING document to review general guidelines for making contributions and having your pull requests accepted.