Looking to grab a copy of the entire scroll-able content of a web page? Aviary, makers of a previously mentioned online image editing tool and Firefox extension, make it really simple: just add the site's URL after aviary.com/.
The speed with which Aviary's Flash-based editor pops open depends on their server load and the content of the site you're trying to paste over—Lifehacker is a "large image", while aviary.com/xkcd loads just fine. But the full-length capture works, allows you to save to the desktop (through the "Export" function) and make basic edits like cropping and text and shape addition. Our initial tests showed some real lag, and a few failures, when trying to open the right-hand tools for "Advanced Editor" or "Image Effects" on larger web captures, but when they work, you're getting a pretty neat Photoshop-lite to play with.
Aviary's URL appending service is free to use, requires a free account to save images online and access certain advanced features.