How to do an inner extrude?

Well, I’m a newcomer to Blender (2 days), and before i started using blender, i used cinema 4d for a couple days (it was fun in my opinion, and yeah).

I’m also enjoying blender, but there are a few things i can get understanding, and trust me, i googled them and couldn’t get any satisfying answers, so yeah…

So, back to topic, how to execute an inner extrude in blender? I found a work around (select, extrude, don’t move mouse button and left click or enter, size, and enter), but it doesn’t work for every shape. Is there a way in Blender 3D default? or do i have to use a plug-in?

By the way, I’m using Blender 3D v2.55 beta (Linux - Ubuntu)

Which shape does it fail to work?

Additionally to the extrude function you have Solidify that can be helpfull in some cases.
In 2.55 it is a modifier, “Solidify” and in 2.49b it is under “Edit Mode” Mesh -> Scripts -> Solidify Selection.