This is not for everybody, but I just got a Rolls PM50s, new for $50. This is a great device for monitoring my vocals directly and blending in a feed from my keyboard rig. It outputs a healthy headphone signal with individual volume controls for mic and line input and has a mic splitter built in. I've used it on a couple gigs and not only does it save my voice and hearing (well, too late for the latter), it gives me a much more nuanced monitoring system than I ever had with stage wedges.
As a keyboard player/lead singer, this is a great solution for me. For singers who move at all (and if you don't, you need to learn to be visually engaging), this solution wouldn't work, as it requires being tethered to a spot on stage (even with the coiled extension cable I was using).
Anyway, for folks who are stationary singers, this is a great solution. It's not a Shure or Sennheiser high-end wireless solution, which are very sweet, but hey, it's $50. Check it out.
And no, I'm not a sales weasel. Just offering some quick and dirty alternatives to nice high end systems.