We've featured a few ways to mass download wallpapers before, but if you have more specific tastes, Pulse will download and cycle through wallpapers based on a keyword you give it.Pulse is a portable application that runs in your system tray, downloading wallpapers however often you want it to, and automatically cycling through them. When you first run it, right-click on its system tray icon to adjust your keyword, resolution preferences, and how long you want to keep old pictures that it downloads. You can type anything in as your keyword, like "cosmos" or "landscape", and it will return all the results matching that search in slideshow form. If you're sick of manually downloading wallpapers yourself, it's a good way to escape the monotony of the wallpapers you already have on your system.
Right now, it's in beta, and only uses one site — rewalls.com — as its source, but eventually it will support more wallpaper sites. It works better for certain search terms than others (for example, "space" returns a strange mishmash of results, while "cosmos" returns a very specific set of wallpapers), so keep trying if you aren't getting the results you want. It's also worth mentioning that rewalls.com is a Russian site, so there may be some loss in translation. A good way to start searching is to translate rewalls.com into English and look at the tags on the right hand side of the page — those should return some good results.
Pulse is a free download for Windows only.