Building circuits with the Pi

I have a lot of experience using Linux and writing software, but I've never really dealt much with hardware / circuits. Now that I have received my Pi, I want to make something that it can control like a home lights controller or a guitar pedal (those are some of my ideas).

My big problem is that I have no idea how to start tackling the hardware side of these kinds of projects. What I'm looking for are resources that would help me build circuits that will interface with the Pi so I can create these cool projects. Obviously I'd probably want to start small with perhaps an LED display or something, but without a clear place to start from, it's hard to "dive in".

I think you could do worse than heading over to some tutorials for the Arduino. (which is not lightyears away from a Pi in philosophy and some functions.
Have a look in general at "breadboard examples / tutorials". I also believe there might be some tidbits about this in the next Magpi.

Nice introduction http://arduino.cc/playground/Main/ArduinoComic
You need to reconcile this with controlling GPIO on the Pi

I also believe there might be some tidbits about this in the next Magpi.

There will be the first part of a beginners guide to interfacing in the June issue of The MagPi.