Yep, I actually own a Pentax lens (labeled Super Takumar), more specifically the M42 mount variant (which is a universal screw mount). You just have to find the right adapter (one place that sells adapters is
www.fotodiox.com ). Its as easy as screwing the adapter on to the lens and mounting it to the camera. You lose certain auto functions though (obviously AF but you also have to physically stop down the lens aperture).
As far as M42 lenses go, there's a whole lot of them (and some weird ones). There's the Takumars that come in M42's but theres also notable Russian lenses (like the Helios-44 58mm f/2 which is known for its "swirly" bokeh and the TAIR-11A 135/f2.8 which has 20 aperture blades). You can also get some Carl Zeiss Jena lenses on the cheap (compared to the modern versions). And there's also a selection of 200mm f/4 lenses that are around $80-$150 if you want some moderate reach.