Eastern European optics will slide right on the siderail without using a separate mount. The optic on my AK (Russian Kobra) is one such optic, as is the one on MTMilitiaman's AK:
All about Kobras:
http://tantal.kalashnikov.guns.ru/bstkobra.html
A simpler and lighter option, but by all accounts a very good one, is the AK IronDot from LaRue Tactical. It replaces the rear sight with a combination sight blade and lightweight optic:
http://www.laruetactical.com/pics/laruetacticalak-irondot/laruetacticalak-irondot.html
The IronDot is probably the most straightforward choice for someone new to AK's, as it is a quality unit, and sighting in is probably simpler than it is for a Kobra. I'm not sure what the going price is for one, though.
The POSP series of scopes is also very high quality, but a magnified optic makes the rifle less usable inside 50 yards or so. A Kobra or other unmagnified optic is plenty usable out to 200 yards if you have decent eyesight.
POSP 4x24 (the max magnification I'd personally want on a 7.62x39mm AK):
http://www.kalinkaoptics.com/detail.aspx?ID=566
Here's that scope on my SAR-1 (back when it wore the factory wooden furniture):
For serious use (as in going to Iraq and depending on a rifle for your life), an Aimpoint Micro on an Ultimak forward rail is the most favored setup---lightweight, sits low to the bore axis, holds zero, extremely rugged, and the battery life is measured in years---but that setup costs more than the rifle.
http://www.ultimak.com/pic38.htm
Finally,you can buy siderail optics mounts that just have a rail on top, allowing you to throw a cheap U.S.-made red dot on there:
The only drawback is that the optic sits
very high (higher than even a Kobra, generally). This is the cheapest option if you go with an inexpensive Wal-Mart red dot, though. One caveat---by most reports, the Chinese made mounts (which use setscrews to secure them on the rail) are lower quality and don't hold zero as well as the Russian made mounts like the one in the photo above (which use a lever and cam to lock to the rail). The Chinese mounts are probably good enough for a Wal-Mart optic, though, especially if you just want to try out a dot setup before shelling out a lot of money.