PCS, I was lucky enough to find a small manufacturer and retailer/installer of fences in the area that sells their "seconds" for a very good price. A 3' high x 8' long section of picket fence of white cedar makes a nice, rot resistant bench that looks very much like the nice benches that Steve shows in his photos. We cut off the pickets above and below the cross-bars, attached the fence section to legs of treated lumber, and it was done. Four years old and they're still looking great.

Plastic (vinyl) lattice is nice too. Non rotting and I've been using a piece for a bench top for about 5 years with no problems. I got it at Lowe's for about $8 and it measures 2'x8'. I think you can get it in 4'x8' too. My entire bench is made of PVC pipe and joints with a lattice top.

I have a really small greenhouse, so I use this as extra space over my taller plants. Most of my seedling trays are on plastic shelving on the other side of the "aisle."

I, too, used 1" PVC and fittings to make my benches (for a PVC greenhouse!). By using combinations of plumbing fittings and the more specialized ones for building projects, I was able to design for my small greenhouse in a way that (hopefully) uses the space well.

My original plan was to use nursery flats as the "bench" top material (good air circulation, no corrosion, no rot, etc.) but am now leaning to getting the "egg crate" white plastic for fluorescent light fixtures (if it's not too expensive. Some of my flats look kind of unhappy with heavy pots on them, even with a middle brace running down the length of the bench and the 16-17" square means there is often wasted space when packing in the plants (the whole greenhouse is only 6' x 6', with one side taken up by three large container plants).

I used water as a heat sink last year and this year I bought a large (130 litre) storage container. I'm going to use it as a low bench for starts, and the tomato plants I want to keep in the greenhouse over the summer.