Debugging Tips and Tricks?

Hi there,

I having been using and abusing WordPress for over 4 years. I want to get better at hacking and developing plugins for my own use and to also give back to the community.

I was wondering what are the best tools, programming languages etc to do this. Especially any advice for debugging plugins that don’t work would be useful.

I have firebug, fiddler, (not that I know how to use them)

Any good plug-in development tutorials out there?