PaakType: Not really a GOG-able game, but I have to give a shout-out to Shiren the Wanderer (originally for Super Famicom, more recently rereleased on DS, with a Wii sequel that came out a week ago). Being a console game, it's heavily stripped down as roguelikes go, with only a handful of verbs and item types, but it plays marvelously--plus, with the SNES JRPG aesthetic, it's easier on the eyes than the typical roguelike. If you have a DS, I can't recommend it enough.
On the PC side of things, Dungeon Crawl feels like one of the easier ones to dive into.
This, get Shiren if you have a Nintendo DS. I picked it up on impulse and couldn't put it down for a couple of months. You even have a system that allows for one of your buddies to rescue you online.
Other than that get Diablo 2, is the king of commercial roguelikes, be prepared to get hooked on it. Fishymancers are fun.