Parts Express is a company that sells supplies for speaker builders. They host a very nice forum for builders, and are overall a pretty cool company. Every couple months or so they post a new speaker design to their design showcase. The newest design as of this writing is a 8" sub with a passive radiator, but all sorts of designs are shown; including stage floor monitors, 5 speaker home theatre setups, line arrays, transmission line designs, just about anything you can imagine.

I have posted articles about individual designs that they've had in their showcase, but I thought it might be a good idea to link the overall page, it's a pretty good resource, and a good representation of what is available in DIY speaker plans.