There's also di:stages.
I finally got around to trying out the expansion set (basically the vertical poles) yesterday.
The recangular bases come one in a pack, with one plastic grippy claw.
The expansion set comes with 4 smaller poles, 4 larger ones and a lot of connectors.
The only real downside is that they cost $5 each so you would have to spend on both packs if you wanted more height. The plus side is that you can build as much as you want.
