Support for reverse debugging in C++
Historical is a great debugger improvement. Unfortunately it was implemented in Visual Studio only for managed code.
Martin Kallman commented
VMWare Workstation integrates with Visual Studio to support replay debugging. Developer's guide http://www.vmware.com/pdf/ws7_visualstudio_debug.pdf
code for implementing efficient deltas, faster and more efficient than xdelta. IDK how it compared to MS's own stuff.