I have never used a debugger with the ability to step backward. I just did a Google search and didn't find much...
I did find this:
http://video.google.com/videoplay?docid=3897010229726822034
Searching for "Bill Lewis debugger" led me to:
http://undo-software.com/undodb_about.html
The ability to step backward implies preserving a lot of state: if you change a variable in one line of code, and then you want to step backward, presumably the variable should revert to its previous value. If you change a register, it should revert to its previous value, and that includes the flags register. If you allocated memory, presumably it should be de-allocated. And so on. It's a big job in general. It looks like this debugger is using copy-on-write technology to actually do all this. I guess if a virtual machine like VirtualBox is possible, so is this.
This is still not a mainstream feature. One feature I have commonly seen is the ability to "set the next line". Say you call a function and it returns a value that surprises you; you set the "next line" to the line before the function, and then single-step into the function. Microsoft Visual Studio debugger can do this and so can gdb (the jump command). This ability is of limited use if the function corrupted the stack. Then, you really need to start over!
Computers have gotten so fast, it's much less painful to start over from the beginning than it used to be. You can just set a breakpoint before the point where you want to go back, and re-run the program from the beginning.
