There are two standards in slotted accessory rails. One of them is the Picatinny, or M1913 spec - the other is the Weaver spec. The official difference between Picatinny and Weaver rails, is that Picatinny slots are spaced 5.0mm apart and Weaver are 3.8mm. However, it seems that vendors with rails which don't conform to either specification will more frequently identify their shoddy rails as Weaver rails. Wikipedia has far more in-depth articles on both the Picatinny and Weaver standards.
One notable addendum is the M3 accessory rail, which is a standard accessory mounting point for pistols. I can't presently find any documentation to support this, but my understanding is that the M3 rail has a single M1913-spec slot. Whenever I manage to get my CZ-40P, which has an M3 rail, I'll verify its dimensions with calipers.
AK and SVD Rails
The standard communist bloc quick-detach scope system is a rail mounted on the left side of an AK, Saiga, PSL, or SVD-family rifle's receiver. The rail is secured to the receiver by two or three rivets (depending on rail type), and the scope itself is secured by a quick-detach clamp on the mount. The scopes' mounts mate to the scope rail, and rise up a couple of inches, then curve to the right in order to partially compensate for the scope being off-center to the left. On a side note, AK/SVD scopes are almost always slightly left of the rifle's true center, and the shooter must learn to compensate for it. Generally speaking, side-mount rails are the best option for mounting optics on an AK, Saiga, PSL, or SVD rifle. Belarusian (POSP, Kobra) and Romanian scopes have been available in the mass market for some time, however there are now Weaver and Picatinny spec scope mounts available for these rifles. Like the factory scopes, the mounts rise up on the left side of the receiver and hook rightward, but feature rails instead of an actual scope on the top of the mount base. Incidentally, the inexpensive scope mounts connect with screws instead of quick-detach clamps. It's also worth noting that SVD rail dimensions are actually different (larger) than AK rail dimensions. Make sure you know what you need before ordering a scope! However, it's also worth noting that all the currently popular Century Arms AK's, Saiga rifles, and PSL rifles are using AK-spec rails presently.
Proprietary Mounting Systems
H&K Claw Mounts
Various H&K rifles utilize a proprietary locking standard, called a "claw mount". This is named for the way that the prongs on either side resemble a claw, and the quick-detach lever causes the prongs to clamp down like one. There are different claw mounts for various H&K weapon systems. There are also non-quick-detach mounts on the market, which tend to cost considerably less than true claw mounts.
Improvised Mounting Systems
Drilling and Tapping
Practically any scope can be mounted to any rifle, by drilling a hole into the gun's metal body and using a tap to thread it appropriately. This is widely frowned upon when done to C&R rifles, as it entails making permanent changes to a potentially historic rifle.
Dust Cover Mounts
SKS and AK-family rifles have both had a variety of 'dust cover' mounts appear on the aftermarket. Unfortunately, there's never been a good, solid dust cover mount for either weapon system. The sole advantage of these mounts is that they're cheap and easy to install. While adequate for mounting a red dot scope for CQB, dust cover mounts are worthless as scope mounts.
Sold by a few companies and for a few different rifles, "scout mounts" are typically small sections of Weaver or Picatinny spec rail which replace the rear sight base on AK, SKS, or Mosin-Nagant rifles. Because they are directly mounted to the same part of the rifle which contains the barrel, they maintain zero quite well. They can also be installed by simply replacing the rear sight with the mount base and sliding a pin through the rear sight's (now empty) pivot point. Because the weapon is readily restorable to its original condition, very few C&R collectors will raise historical integrity questions to the use of a scout mount on a rifle.
-- SeanNewton - 07 Dec 2007