Here is a javascript bookmarklet that will save the current page to the Internet Archive. This opens a new window so you can continue browsing while it's being archived.

Drag this link to your browser's bookmarks: Internet Archive This!

Here's the code for the bookmarklet if you prefer to copy/paste/tweak it:

javascript:var w=window.open('https://web.archive.org/save/'+location.href,'','scrollbars=1,status=0,resizable=1,location=0,toolbar=0');