OpenBUGS is the open source program for Bayesian modelling that was developed from WinBUGS. OpenBUGS is not currently under active development, but the current version and all previous versions are provided here. The content on the main OpenBUGS site is out of date, but is in the process of being moved here.
OpenBUGS looks very similar to WinBUGS, and is just as stable and reliable, having been validated on a wide range of examples. However OpenBUGS has more advanced functionality for MCMC sampling, and unlike WinBUGS, runs natively on Linux as a command-line program. This list documents the differences in full.
The latest BUGS developments are focused on MultiBUGS.
See the paper The BUGS project: Evolution, critique and future directions for details of the history of the BUGS project.