I have heard a couple people absolutely rave about the Fujinon EBC 100mm f/2.8.
Sadly, one never appears to become available on e-Bay.
If you are willing to look at 135mm lenses, I've a bit to say about a couple Pentax Super Multi-coated Takumars that may be of interest.
Pentax Super Multi-Coated Takumar 135mm f/3.5: Hard to beat as a carry-around general-purpose 135mm telephoto. Not the fastest lens, obviously, but very sharp even at f/4.5 and it performs very well focused at near distances. Contrast is very high. Very well-built with smooth but well-damped focusing and very nice handling (takes 49mm filters!). It's bokeh is not quite as good as I'd like. Even so, I think this may be among the best values in the M42 world because it can be had for 45 USD on e-Bay.
Pentax Super Multi-Coated Takumar 135mm f/2.5: If you are trying to seduce a beautiful woman...take her picture with this lens.
It excels as a portrait lens. Nice, fast aperture, and very good to excellent bokeh. It's build quality is also excellent and its focusing action is similar to it slower brother described above. It's slightly harder to handle as it's heavier and accepts 58mm filters. I've only used this at its wider apertures (say, f/2.5 - 5.6) and it doesn't seem to be quite as sharp and contrasty as the f/3.5 lens and it's near-focus performance is, similarly, not as strong in that range. Again, that isn't unexpected for a portrait lens. And it really, really does great work for portraits. Figure 70-90 USD on e-Bay.
I have heard, however, that above f/5.6 this becomes an extremely sharp lens and contrasty lens - more so than the f/3.5. I just haven't put it through its paces in that aperture range yet.
Hope it helps.