Trackback is a Movable Type-originated spec for providing inter-blog links (and a simple form of threading) that completely and utterly failed to live up to the hype and is now essentially Spam fodder (pretty much like Referrers, but those are actually more reliable).
Resources:
- TrackBack: A Tragedy in Three Acts - brilliant.
- HaloScan, a free third-party Trackback and commenting system.
- Trackback Specification
- Jannis' Trackback Lib (a PHP pinger)