Something I've been planning to do for quite a while was making the jump to a dual monitor setup. I finally took the plunge and after about a week, I already can't live without it.
The extra screen space is fantastic and it helps me monitor news sources and my sites much more easily.
The best part is that it isn't expensive to set it up. First, I purchased a used 17" Dell M770 for $45 on eBay (I love eBay!). It was a local pickup so no shipping fees. Next up I found a GeForce FX 5200 graphics card with VGA and DVI output. My graphics card came with a DVI to VGA adapter (this is where you'll plug in the second monitor), so there was no extra cost there. I was in a rush and didn't shop around so I spent just under $100 for my card at Office Depot. But, I just checked eBay and they have the same card going for about $60.
So, for less than $150 you can fire up a second monitor and improve your productivity.
I'm already thinking about going to three monitors.