I just made a reservation using OpenTable.com at Tavern on Rush in Chicago for next Saturday night. Love it. No looking up phone numbers, no calling, no fuss. Dinner at 9 for two, confirmed immediately by email just like that.
More than 8,500 restaurants use the service internationally - including several in Grand Rapids. I've used OpenTable to make reservations at both Six.One.Six and Reds on the River in the past.
They even sent my other party an invitation at my request. Fun!