I can't praise this program enough! If you're a modeller/skinner, you absolutely have to download it. It's called Deep Exploration (previously called 3D Exploration), and it's created by New Zealand's, Right Hemisphere. I'm currently skinning my male model, and I'm using Photoshop to do the 2d work, while viewing the model in Deep Exploration, and I absolutely love it.
It supports PSD files, so you just save your texture in PS, and when you click in DE, it'll update the texture on your model automatically! It's got a gazillion display and lighting modes, in openGL, direct draw, and you can even view your animations in it too! I've found tonnes of other useful things that DE offers, but I'm sure I haven't even scratched the surface yet - it's much, much, much more than just a 3D model viewer.
There's a 30 day trial version available so you darn well should head over there and download it. :) Once the trial is over, some of the functions are disabled, like exporting, batch conversions, raytrace rendering etc but I'm sure it will still be extremely useful as part of your working environment. Head on over and get Deep Exploration.
And since I have your attention, callibrate your monitors..(thanks Beatsta)!