This is a shelf I built using Kee Klamps and 1-3/8" top rail fence post.
I purchased the Kee Klamps from Simplified Building Concepts, you can also get them from Grainger and McMaster-CARR prices are about the same anywhere you go (read: expensive) but Simplified Building seemed to have a better selection. The Kee Klamps are awesome and I could come up with a ton of things to build with them, but there a bit cost prohibitive to build with. All in all though they work quite well. I couldn't find the weight bearing specifications of the Kee Klamps but I was able to stand on the middle shelf (~160lbs) without failure. This was an expensive project mostly due to the use of the Kee Klamps, but they sure are cool.
Kee Klamps Parts List:
4 x Type 20-6 Side out Elbow (1")
8 x Type MH50-6 Male Single Horizontal Swivel Socket Member (1") (one for each corner of the top and middle shelf)
4 x Type 61-6 Flange (1")
~$131.34 (includes shipping to my house)
Pre-cut 24"x24" plywood from Home Depot (for shelves)
4"x2" for the bottom shelf bracing
1"x1" for top and middle shelf bracing
1-3/8" top rail fence post from Home Depot
2" Casters (need to be able to pull it out to get the network equipment behind it)
Miscellaneous nuts, bolts and screws
Finish of your choosing (I used clear polyurethane)
It's probably best to build the base first since it's.. the base and will affect the placment and size of the other shelves.
I used 4"x2" for the bottom shelf, I could have gotten away with 2"x4" but I wanted to be sure that I had sufficient room for the casters and the flange bolts; plus it gives it a nice solid base. They were attached to the plywood with wood crews - 9 on each board.
I used 2"x4" casters from Home Depot, I don't plan on putting much weight on it but the casters support something like 120lbs. make sure you get casters that will hold what you plan to put on them. The casters were attached with bolts to the 4"x2".