I did exactly what you're talking about several years ago, shortly before 802.11b came out.

We basically had a pair of wireless routers, one in each building, connected to directional antennas on the rooftops. Because this was pre-802.11b, it was only 1.5mbps, but it worked. Very simple to set up, and not terribly expensive. At least it was a lot less expensive than leasing a line from the phone company.

Nowadays you can do it with much faster routers. In many cases it's simply a question of getting a pair of off-the-shelf routers where you can unscrew the little antenna and attach a coax cable running to the big antenna on the roof.

When we did it several years ago, the company we got the gear from was "Proxim", and I'm pretty sure they're still in business.