Canon cameras also offer infra-red based camera slaving; you can either get a wireless flash transmitter, or use one mounted in a hotshoe to trigger others; you can even set relative ratios of flashes between several units, and have flashes "listening" on different channels. It's pretty nifty.
Off camera cords are pretty handy accessories too, particularly for macro.
And yes, running around with a single fixed focal length lens helps you learn a lot about using light and framing/composing pictures without the 'distraction' of a zoom - a 50mm lens is pretty much perfect for this, and you can pick up some really amazingly sharp, fast lenses like this for a fairly insignificant amount of money. That said, once you've learnt a bit about this, zooms are pretty handy things to have attached to your camera. There's something to be said for spending a day (or more!) taking photographs with just one lens and no other accessories.
In an ideal world, I'd drag around a bunch of cameras with several primes permanently attached.
As you said, super-wide angle lenses are a bit "gimmicky" and actually extremely hard to use well; mine seldom comes out of the bag. But when you need to get a whole room in and don't have any way of "backing up", they're invaluable. The relatively large depth of field effect you can get with these lenses is also pretty great in landscape photography. If you do it right...