Table
A good steel shop table is a must-have. You can customize it to fit your desired working height and the dimensions of your workspace and projects. The table isn't about being fancy. It's about creating a steel work area that's perfectly level -- even if the floor is not.
Rack
Whether you're a neat-nick or mad professor in your workshop, you'll probably agree that a degree of organization can make your projects go more smoothly and quickly. Making an incline rack for stock material is a big help. Essentially, you're making a rectangular frame out of square tube or channel stock, leaned against the wall about 10 degrees. Weld a few perpendicular pieces to hold long stock like shelves. Gusset them if you'll be storing lots of heavy stock.
Deck Extension
The fabrication process usually turns larger stock into smaller pieces. You've got to cut the stock somehow. Whether it's with a plasma cutter, horizontal bandsaw, chop-saw or torch, hold the stock firmly as you cut it. Decks for commercial saws frequently aren't long enough. And if you're trying to cut 20-foot stock by laying it across an 8-foot deck, it's liable to bend, stress your blade and perhaps create a real danger. Custom deck extensions support your long stock while you're cutting, save your blades and ensure square cuts.
Miter Jigs
There is an infinite variety of jigs -- anything to hold your work as you're welding. A miter jig is a common example. You'll likely be making a lot of 90-degree miter-welds. Start with one, but tack it to a steel plate you can clamp to your table. Get the jig perfect, then clamp future miters to it for fast insurance.