web page screen capture utility?

hello,
are there any command line utilities that would let me get a screen capture of a web page? ultimately i want to build an archive of web page snapshots and i'm looking to build a script that would automate this.

so far the only thing i can think of is to have a firefox window open, then do a firefox -remote URL command to load the page i want into firefox and then use xwd to capture the firefox window to an image.
[although i'm still not sure how to get the firefox window resource ID so that i can grab it with xwd] however this is still fairly problematic because i won't know if, or when, the page has finished loading.

is there a better utility for doing something like this?
really appreciate any help.
thanks,
ken
.