When I was writing about bookmarking tools that store cached copies of web pages, I completely forgot about Spurl.net. Luckily, roxomatic's Social Bookmarks review pointed out that Spurl caches bookmarked pages, as well as having many other useful features (including counting how many times you click on each stored bookmark). Even better, Spurl doesn't overwrite old cached pages, so you don't have to use a 'unique dated URL' hack. Stored pages are linked using a simple URL scheme and are divided into "your" and "others'" cached versions.
Spurl also syncs nicely with del.icio.us (in both directions).
Here's my customised →spurl← posting bookmarklet.