In recent years, router manufacturers have begun offering routers with height adjustments that can be accessed through the base, when the router is hanging upside down in a router table. This eliminates the cost and complexity of incorporating a router lift into the design to control the height of the bit. These new routers have that built-in.
Inspired by these, a number of writers have published designs for router tables using these new routers that aren't really tables, they're router table tops, meant to be attached to an existing workbench.
Bench-Mounted Router Table
Stow-and-Go Router Table
Stow-Away Router Table
When I was building my own Real Woodworker's Workbench, I had to buy a second router. I decided to buy one with an integrated lift, and to use it to build a router-table attachment for my bench, similar to those described above.
It took me longer to get around to it than I had planned, but a few weeks ago I finally got started.
I made it out of a 24"x48" piece of 3/4' Melamine-coated particle board, with a 3/4' backing of Medium Density Fiberboard (MDF), using a commercially purchased insert plate and fence. Melamine provides a slick, smooth, easily-cleaned surface, which makes it a pretty good choice for a top. 3/4' isn't enough to prevent bending, though, and the fiberboard core isn't as dense as MDF, hence the MDF backing.