There are a few tools I use in development that - once I started using them - I've found I just can't live without them anymore. I'm often surprised when I encounter other developers that don't use them, or use very primitive versions.

Here are a few of them:

• A good editor. I use kate, but it is of course, a matter of opinion. This has been discussed many times.
• Version control. I use cvs. subversion is meant to be better. One day I'll get around to switching to it.
• grep (or similar). How else are you supposed to easily find all the calls to a specific function across many files?
• ERDs, DSDs, etc. (i.e. diagrams of your database) - I use the fairly lightweight (and proprietry) DDS. I really can't understand how people manage to keep the whole DB schema in their head.
• Testing tools - only fairly recently becoming essential for me, and I *could* probably live without them at the moment. But I seem to be becoming more and more reliant on them.

What other tools can't you live without?