Omniscient Debugging is an idea that looks very very sexy. In short: it's a complete trace of a program after it ran - all variable assignments, function calls, exception throws/catches, etc. Once your program finished, you can retrace its whole execution history, step by step.
Imagine that your program failed. So, you have to run it again, fill in lots of "print" statements, perhaps even firing up the debugger and starting throwing breakpoints in. With ODB, it's not necessary. Even for multithreaded programs, it provides a completely deterministic run history that can be used (and in fact, was displayed in action) to find bugs very quickly. People who use it swear by it, hotly claiming that it shaves off huge amounts of debugging time.
Now, that catch is - this ODB thingie was developed for Java, since its virtual machine allows a simple & relatively efficient implementation of an ODB to sit on it. It probably isn't too applicable to compiled code.
But I guess Perl can have support for this as well ? Perl 6, especially, which also runs on top of a VM... Can Parrot support this ? The Perl 6 compiler, perhaps (inserting "timestamping" instructions before/after assignment/call instructions)