I've just created a home for the Memento Browser for Android, a project I started working on this past summer. The free Android app allows you to view older versions of web pages by merely selecting a date. The browser uses the Memento protocol to find archived versions of the page and displays whatever page is closest to the requested date.
For example, the screenshot below shows the browser viewing cnn.com:
If you wanted to see what this page looked like on Sept 7, 2007, you could select that date, and in a few seconds be looking at this archived page from WebCite:
Note that the page displayed is actually one day later than the requested date. That's because the browser was not able to find an archived copy on the exact date requested. The browser is only displaying archived copies from Internet Archive, WebCite, and a few other archives. While they have a huge amount of the web archived, they certainly don't have everything archived.
You can download Memento Browser here. I am working on an iPhone version of the app with a colleague of mine, but I don't have an ETA for it yet.
If you don't have an Android device, you can still download the MementoFox add-on for the Firefox browser which does the same thing.
Finally, you can watch a demo of the browser in action here.