This plugin is geared towards people making VMF maps with Sketchup.
Here's what it does, and how it works.
It's very helpful if you're building VMFS for l4d as outlined in this great l4d howtop by zapwizard.
What you do is simply create your building out of ungrouped faces, cut in your doors, windows, etc.
When you are done, use the select tool and TRIPLE CLICK your geometry. (This is important!, for reasons explained in a minute)
Right-click on the selection and you'll see in the context menu "Convert To Extruded Groups"
If you select it, every face in the selection will be grouped, and extruded (pushpulled) 8 units. Materials that existed on the front-back faces will be applied to the new group, and the sides have their materials taken off (so you can nodraw them).
Now it's not perfect, edges aren't mitered, so you'll have to manually touch those up by opening each one and pushpulling them, but it still saves a load of time.
Update 2011/03/14 - Added support for areaportals. If you paint any face with an area portal material (a material NAMED "tools/toolsareaportal"), it will be applied to all faces in the extruded groups and they will all be grouped together so you can easily Ctrl-T it in hammer to a func_areaportal, etc. Note that I am applying the material to the backside as well as the front, a no no for hammer, but if I leave it unpainted, it's solid and you can't see through them, so it's strictly for convenience
Update 2011/03/14 Added an option to set the extrusion depth. Under the Plugins menu, select "Convert To Extruded > Set Depth"
Here's a video of it in action.
Here's a quick workflow for mitering roofs and angled walls.
WHY TRIPLE CLICK IS IMPORTANT!
Because adding existing entities to other groups bugsplats in Sketchup 8, I need to recreate the face in the new group, the old face, AND EVERYTHING CONNECTED TO IT WILL BE DELETED. So if you only click one face in your building, the rest get blown away, but hey, that's what undo is for.