One that I really like is http://www.digitalblasphemy.com
It is a pay site, at least to get to the widescreen wallpapers, most of which are in 1920x1200 or 2560x1600. However, the rates aren't all that bad, and it's all-you-can-download during your membership period. I paid $12 US for a 90-day membership and downloaded every 1920x1200 'paper he had (130 images at time of writing). I don't mind supporting the poor guy for less than a dime an image. :P
As far as the content goes, it's all digitally rendered scenes - some fantasy, sci-fi, nature, etc., but it's all original and pretty high quality imo. Anyway, check it out for yourselves, and I hope you enjoy it as much as I did.
I did the same. His stuff is very high quality and I dled all his wallpapers at the highest resolution. If I want to use one, I'll just photoshop it down to 1680x1050. This way if I upgrade to a larger monitor down the road which is inevitable, I'll still have the higher resolution wallpaper.