Remap your keyboard in X! Ok, so not really a tool, but one of those things you wish someone had told you about earlier. Excellent for getting easy access to all those neat unicode symbols (generated with gen sampler.py).
gucharmap is a nice tool for finding Unicode encodings for whichever character you're looking for.
Another approach to binding special characters is to use the compose key. For example, I use Xmodmap for Greek letters, and XCompose for accented characters.