If your into illustrated Wallpaper here are some great ones: 25 Creative Illustration HD Wallapers by Blurburger + 9 Free Icon Sets

Examples:

If your a gamer then here are some good ones:

Top 10 high-res wallpapers of upcoming games

Here are some examples

Gran Turismo 5

Battlefield Bad Company 2

OK I just got this pic in a email and it was large so I saved it and its now my background. If you like it and want it save it from here (I reduced it to smaller here but it will save full size)

Wallpapers? What's that? I never see one on my computer

I never see my desktop. I use TC (Total Commander) and not Explorer and that is always open. If I don't have it on my quick luanch bar. it's in TC. Best "file manager" out there IMHO

P.S.
So I lied I do see my wallpaper when I reboot or lock my PC but that is not so often either.

So who changes their wallpaper for the different times of the year? I admit I do. Here are some really nice Christmas Wallpapers (all free): Noupe.com

Here are some of the examples they show (all 1600 x 1200):

and best of all they have DUAL, TRIPLE and QUAD Wallpaper

Here are some great pictures that when you click on them they are large enought to use as wallpaper:

The Best HDR Photographs of 2009

If your looking for James Cameron's AVATAR the movie computer wallpapers then check this thread out:
FREE Avatar The Movie Wallpaper

50+ Free Valentines day wallpapers for your desktop

But honestly you can use them all year long since they are mostly hearts. Oh yeah I am changing my laptop walpaper today (since my son changed it over the weekend)

It you happen to have a love for Typography then here are some:

45 Free Inspiring High Quality Typography Wallpapers

deviantART Typography

Or even if you just want to play with the pictures you can - the ideas are endless............