I recently needed to find out a list of modules which used hook_cron. I have SSH access to my server. Instead of writing a module (or using devel to execute some PHP) which returned the result of module_implements, I turned to grep. Simply cd into your modules folder (can be the Drupal install, but you end up recursing through unlikely folders) and run this:
grep -R "^function.*_cron" *module
That will recursively search for all files with any lines that start with function, have anything between that occurrence of function and _cron. You can replace _cron with any hook name you like. I hope this is of use to someone else too.
The Drupal face came from a derrivative of Steven Wittens Wallpapers. If you'd like me to take it down Steven, please let me know - I didn't think you'd mind as there doesn't seem to be any CC license notes on the page in question. Nice wallpapers by the way!