VMware just blogged about VAssert (programming guide – PDF), a new API debuting in Workstation 6.5 (beta) that uses the Record and Replay functionality they’ve been talking about for some time now. VAssert (Virtual Assertions) is a relative of your standing programming ASSERT debugging tool, but by delaying assert-checking until later when the exact machine instructions are replayed, it can be “very fast”.
VMware engineers Weiming Zeng and Min Xu demonstrate VAssert within Apache here.