I found this lens at a thrift store for $15.
It's an Adaptall-2 lens, which means that if you have the proper adapter, you can use it with just about any SLR mount out there. This specimen came with what looks like an M42 adaptor, marked 'For Fujica.' The mount-end cap is also marked 'For Fujica.' Maybe it's for Fujica? :-)
I tried to use it with the M42/Pentax K-mount adaptor I have, but an exposed lever (only exposed about a milimeter) caught in one of the mount screw holes on the camera. The camera's a Pentax *ist DS. My guess is that Fujica made a variation on the M42 mount which included this little lever.
Luckily, I have another nice Tamron Adaptall lens I got at that very same thrift store a while back, a CF 80-210/3.8-4, and it has the Pentax K adaptor. Swap the adaptors, and voila! 28mm goodness. Thanks, Tamron!
Unfortunately, however, just like the zoom, the 28mm's aperture can't be used in the 'auto' position. That is, even though the aperture ring is marked with an 'AE' for auto-exposure, and even though Tamron says that should let the camera set the aperture, it just doesn't work. I'm not sure if there's another better K-mount adaptor or if it's something my *ist DS wants that the lens can't provide.
(Update: Well, yes. There's a better K-mount adaptor. According to this Tamron FAQ, there are three Pentax adaptors: Screw mount, K-mount, and KA-mount. The KA mount allows metering and aperture priority, where as the plain-vanilla K mount does not. Now that I know this, I'm going to order two of them. I'm also going to add another page devoted to this topic.)
So I'll go out on a limb here and say that Adaptall lenses won't really be useful in exposure modes other than Manual, at least on an *ist DS. This isn't a problem for me, because a manual multicoated 28mm/2.8 lens is a nice thing for $15.
But there's a light at the end of this particular apeture: This lens has an A/M slider, which lets you do stop-down metering. You can use this slider to set the aperture to fully-manual mode, and magically Av mode will work (albeit only in center-weight or spot-meter). It's tremendously useful. You can put the slider in A to compose and focus, and then M for metering. Much easier than moving the aperture ring (and subsequently forgetting where you had it set).
I haven't shot much with it, but will update this entry as I have more experience. I anticipate using this lens for stitchable panoramas and photojournalistic candids.
See also this page of notes on using manual lenses with Pentax *ist DS cameras.
Some technical info: 28mm/2.8, minimum focus distance is marked 10 inches, but the focus ring goes down to closer than that (I'd guess 6 inches, which is pretty impressive). Infinity is just past the 10 foot mark, probably more like 15 feet. Near to far focus is a turn of about a third of the circumference of the lens. Filter size is 52mm. Minimum aperture is f/16. Aperture settings are in half stops.