OpenBUGS has been compared to WinBUGS 1.4.3 on all of the examples in the OpenBUGS manual. A graphical summary of the posterior means from each parameter in the examples is in normalized mean difference. Each plotted point is (WinBUGS-OpenBUGS)/sqrt(WinBUGS SE2 + OpenBUGS SE2). A standardized difference in the posterior means of the parameters, 2*(WinBUGS-OpenBUGS)/(abs(WinBUGS)+abs(OpenBUGS)) is in standardized difference.
The code to perform the comparisons is in the OpenBUGS repository on www.sourceforge.net. Instructions on how to execute the code are here: compare WinBUGS/OpenBUGS