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Being able to step backwards through code while debugging would be nice...
Here are some google tech talks / presentations...
http://www.youtube.com/watch?v=LpfmKIxusZY
http://video.google.com/videoplay?docid=3897010229726822034#
And there's a debugger here (which I should try out sometime...):
http://www.lambdacs.com/debugger/debugger.html

Still, I haven't seen this being available with many different platforms.

http://www.lambdacs.com/debugger/debugger.html/

Seems like this has been done before, at least for java apps...

Take a look at
http://www.lambdacs.com/debugger/debugger.html I'm sure a Hindsight sales person would (correctly) say this isn't a complete solution, but its the closest thing I've seen before this article.

Debugging backwards in time. See the Omniscient Debugger for an implementation in Java. Instead of re-executing the program a thousand times, each time setting breakpoints and watchpoints in different places to get nearer to the root cause of the problem, this debugger completely records all key events and lets you view the complete program state at any point in time.

Here is an experimental Java debugger which is able to remember every event happening in a program:

http://www.lambdacs.com/debugger/debugger.html

During a program run, it collects "time-stamps" for every event happening in the VM (using bytecode instrumentation). This generates a (potentially huge) trace of the program which your can then inspect in a GUI. The UI is quite powerful (although a bit complex).

Along with some polishing of the UI, the only thing it lacks is some kind of language to query the trace: currently you can only use the UI, which can become a bit tedious. It would be nice to have predefined queries to find common mistakes (deadlocks for example; yes, it works with multithreaded programs).

http://www.lambdacs.com/debugger/debugger.html

This guy gave a presentation on "Omniscient Debugging". Apparently the tool can state changes in program state (including those of multiple threads), starting and terminating at a specified times, so you can basically go forward and backwards across an event and analyze it. It seemed pretty nifty to be able to be able to "rewind" a bug event at will.

I believe you are referring to the Omniscient Debugger: "A software tool written with Java[TM] technology allows developers to step backwards through the execution of a program to determine where and how programming errors occurred. By recording each state change in the target application, it allows the developer to navigate "backwards in time" to see what the values of variables and objects WERE, enormously simplifying the task of debugging programs. "

I haven't used it myself, but this debugger for Java allows you to record the execution of your program and play it back again and again to see what's happening, who's setting what variables, etc http://www.lambdacs.com/debugger/debugger.html It appears to be free (as in beer), so there's a bonus for your right there!