Four from Compiler Science:
- Does a program ever access an uninitialized variable.
- Do two context free grammars describe the same langauge.
- Does it make a difference if parameters to a subroutine are passed by reference or by copy-result
- Deadlock determination in parallel programs.
Actually almost every question of the form "Does a program ever do X?" is equivalent to the halting probelm. So the above might be considered too close the the halting problem to be interesting answers ot this question.