To get a # sign on an Apple keyboard you use the Option (or Alt) key + 3. This seems terribly klunky to me, and # is of course used quite a bit in programming and sysadmin work.
This hack remaps another key on the keyboard to produce the # character. I chose the funny squiggle that’s to the left of the number 1 key (§). This is the Section sign, used in document formatting. Just create a file at ~/Library/KeyBindings/DefaultKeyBinding.dict which contains the following:
{
/* this will make all § turn into # */
"\UA7" = ("insertText:", "#");
}
Any app that uses Apple’s Cocoa interface widgets for text input will pick this up after being restarted. There are some that don’t (perhaps TextMate? Not checked that one so if you know, please comment).
A lot more information about this is available at this excellent page on the Cocoa Text System, including some other neat hacks. Enjoy!