The Internet Archive announced yesterday a new Firefox add-on for using The Wayback Machine. It’s not an extension per se, it just adds a little icon to your upper right search bar in Firefox.
(You can see more add-ons related to the Internet Archive at Jeff Kaplan’s page. Add-ons incldue searches for the Internet Archive in general and searches for NASA images and The Open Library.)
Enter the URL of a site in which you’re interested and you’ll get a Wayback result showing all dates available. In the case of http://www.yahoo.com, which I tried to check first, I got archived pages going all the way back to 1996.
One caveat, though — this is for site URLs only. You can’t do keyword searches this way. (Didn’t you used to be able to search the Wayback Machine by keyword? I think it was a pretty long time ago.)
I wish this was more of a full-blown extension — where you could pull up current and former versions of a page, highlight differences, etc — but it’s nice just to have that in my search bar. If you haven’t used The Wayback Machine in a while, go take a look at the advanced search page. You can now filter by file type, compare two versions of a page, and even get a PDF output of an old page version (this feature is in beta.)