I've had multiple monitor setups for a long time, mostly for my recording studio (which is now just a bedroom in my apartment).
But as a Macbook Air owner I'm down to one 23", with a 24" on my studio PC.
Recently I've craved more monitor space for dev purposes. I'm working on a few things, each of which requires a lot of testing, monitoring console outputs, multiple browser windows, and of course a few windows of code (someone tell me I'm doing it wrong).
After spending a few hours looking for another 23" monitor--and finding an HP refurb for $89 if I could just make it to Best Buy in Fargo to pick it up--I stumbled across some software that makes it relatively simple.
Avatron (and a bunch of others) makes a simple app that works over wifi to display a mac screen on a PC screen. For $20, I'm now using that 24" PC monitor for displaying 4 console screens--extending the Mac screen rather than simply replicating it. It's exactly what I needed. Yes, I could get an adapter, but any decent one costs somewhere over $60.
There are other methods (TeamViewer could work) but this was simple and works well enough for what I'm doing. And it saved me about a hundred bucks.