The SKS is a 10 shot semi-automatic rifle with non-detachable magazine chambered for the ubiquitous 7.62x39 cartridge. It was designed in Russia, and fairly quickly superseded there. It continued to be used by backwater Communist client armies around the world for quite a few years. It has a reputation for excellent reliability and indifferent accuracy -- not a gun to put five shots through a single ragged hole at 100 yards with. The SKS was never designed as a selective fire weapon, and thus is not a true "assault weapon". Surplus and new production Russian and Chinese SKS' were imported into the US until the Clinton "assault rifle" ban a couple years back. Manufacturing quality reportedly varies from country to county, with Russian made SKS' with milled trigger group and threaded barrel supposedly the best. They were quite cheap. They can still be had, but prices have shot up quite a bit -- check Shotgun News or Gun List. Look to spend somewhere between $200 and $300.
Parts and accessories to make your SKS uuuuuugly-cool looking
He's got an SKS FAQ and an online SKS manual.
Another SKS site
Red Star Arms carries the Mojo peep sight for the SKS (and AK).
So does Lyman. Most replace the existing rear sight. Lyman's mounts on the receiver (requires drilling and tapping screw holes). This is what I'd like to try.
Disassembling (and reassembling) your SKS
SKS forum at Kalushnikov. (Yes, that Kalushnikov.)