Self timer. Although I keep meaning to get hold of one of those "universal" ones which fits a standard cable release socket so I can use it with the ETRSi.
I don't think I've ever used it for the classic self-portrait, but I do use it extensively for long exposures when I didn't bring the cable release and tripod (I just use a handy rock, bench, wall, whatever as a support).
Pretty much all of the better Pentax SLRs have a locking shutter release - the original three K bodies (but not the K1000 for some reason), the MX, ME, ME Super, Super A, and obviously on the P30 series and later if you switch the camera off the shutter won't fire. DOF preview is a bit more vague - it turns up on some bodies but not others. The KM/KX/K2 have it as does the MX and LX, but nothing else until the Super A and then the P30 series.
I don't tend to use it on the cloth shutter bodies as I don't like leaving them wound on for more than a few minutes, the springs controlling the curtain tension can be stretched if left cocked.