Xapian is a probabilistic IR framework, released under the GPL and written in C++.
Xapian was developed from the code of OpenMuscat (also known as Omsee), which was developed by Brightstation Plc to replace the original Muscat sources. Muscat involved people such as MartinPorter. Muscat was a fundamental part of EuroFerret and WebTop.
Xapian's Indexes are described here.