Zedge is a great, all-in-one option with a bevy of wallpaper choices. There are fresh wallpapers served up all the time, both of the live and static variety. Pretty much any genre you can think of is here, with comics, nature scenes, landscapes, and urban settings among the choices.
The ads are a little annoying, but it's what you'll have to put up with in order to take advantadge of all these freebies. Zedge also goes beyond wallpaper, offering ringtones, icon packs, and even game downloads. If you want a free and easy-to-use option, this is a great place to start.