Here’s a description by Middlespot:

Have you ever wanted a blank workspace on the web where you could paste anything you like in any order you want? A mashtab lets you do just that.

Collect relevant websites, links, images, videos, files, widgets, and just about anything in one central spot, access it from anywhere, or even embed it in another website.

You can annotate, cluster, layer, pan, resize, tweet and share the content you’ve collected. A mashtab makes a great start page or personal collection of interests.
Mashtab Web Site

Screenshot of mashtabs.com

Mashtab Interface

Developers that are comfortable with Firebug or the Web Developer Toolbar plugins for Firefox will find the interface familiar. There’s a toolbar at the bottom of the page that lets you edit your particular tab. There’s also a search engine above, powered by BOSS, that lets you find new information for your mash tab.