SlimPage is a Javascript library used to produce an effect similar to Lightbox. The difference is that Lightbox is used for image galleries, and is not useful for other sorts of data. SlimPage takes that idea and, based on the code of a Lightbox clone, Slimbox and the Javascript framework MooTools, uses the same effect to display webpages.
Click here to launch the SlimPage lightbox effect.
Click here to view the same page normally.
The only difference between the above links is that the first contains the directive rel="lightbox", while the second doesn't. If you include the SlimPage library, all links with this directive will launch the lightbox effect. This processing will also happen in any page loaded within the lightbox, so you are able to have whole sections of your site navigable in this fashion. In addition, if the user has javascript disabled, the links will still work as normal.
Note: Unfortunately, SlimPage will only work properly when hosted on and gotten via a webserver; the Javascript AJAX functionality does not work properly when attempting to get pages from the local filesystem.