I had a workbench in the garage that came with the house and was made out of an old solid core door and some 4x4s and 2x6s. It was 7'x3' and massive, took up a lot of space in the garage, and was mostly full of junk and wasted space. I decided rather than a deep workbench, I wanted one that was pretty shallow, but very long.
So I built a workbench that's 18" deep and 20ft long. I won't actually use it as a workbench except for small things, mostly it will hold tools, chargers, a bench grinder, a vice, etc. on top. Underneath there will be spots for my tools drawers, air compressor, shopvac, a couple shelves, trashcans of chicken food and organic fertilizer, etc.
The work surface
I put channels in a 2x4 frame to hold 1/2" OSB, and then the masonite is glued on top of the OSB and edges of the 2x4's. That way you can't see the edge of the OSB, just the face of the 2x4 and 1/4" masonite. In this video underneath you can see the OSB and short blocks of 2x2 I used at each corner. This allowed me to screw orthogonally cross-grain to cross-grain (no toe-screwing, or screwing into end grain).
The lower shelves
For the lower shelves I wanted to just use 2x2s and didn't want to go to the effort of doing the channels+OSB+masonite. So it's just 1/2" AC plywood sitting on top of the 2x2 frame. The 2x2" have lap joints at the corners (hidden lap joints on the front), which you can't really see in the video because they are hidden by the shelf surface. The plywood is attached via screws from underneath.
It is constructed out of:
2x4 - frame of top surface and most legs
4x4 - leg on the corner by the inside door (other corner is attached to the foundation)
2x2 - frame of lower shelves, corner support for work surface framing
1/2" OSB - main structure of work surface, hidden
1/4" masonite - surface of bench
1/2" AC plywood - lower shelf surface
Everything is attached via screws except the masonite surface is glued down
No screw heads are visible from the front/top/ends, only if you get down and look underneath. :)
The rear rails of the top work surface and the lower shelves are attached to the wood wall at the 16" on center studs.
The side rail of the top work surface is attached to the foundation with masonry anchors (it's not going anywhere).
The two lower shelves are where they are to provide additional support for the legs. There is one leg that does not have additional support, it's pretty solid but I might decide to add a brace there.
I did the end corner miter by hand, but didn't come together perfectly, so there is a small gap I will need to fill with wood putty or something :( This is one of the few times I wish I had access to a nice miter saw.
Because I don't have a table saw, to cut the channels for the OSB I constructed my own fence jig out of OSB and 2x4s so I could do the cuts with my circular saw. This was very time consuming, a table saw (or router/rotozip) would have taken way less time. It worked, but the cuts weren't as good as a table saw would be either. But luckily all those cuts are hidden under the masonite :)
All screw holes were predrilled and countersunk. Most the time I was using a drill to make the hole, a counter-sink bit in my driver, and another driver for the screw. It was pretty time consuming, but way faster than using only one drill (which is what I used to do before I had my nice Bosch tools).
I considered using marmoleum for the surface, but even using remnants it was going to cost a lot more than masonite.