Use one of these kits to build either a workbench or storage shelves. The Simpson connectors are unbelievably well-engineered and durable. You supply the 2x4's and plywood, the kit contains everything else, including self-tapping screws.
This is a great deal - $20 cheaper than Amazon. I used one of these to make an 8' x 8' shelving unit for the garage. Perfectly square and absolutely rigid due to the design of the connectors.
Spend $5 more at homedepot.com and get free shipping.