The Trioplan line of lenses is quite popular for it's "interesting" bokeh. The 105/2.8 is the one I have and it does the soap bubble bokeh (onion ring, bright edge, etc) very well. The 50mm version has similar but it's not as obvious, but significantly cheaper than the 105mm. You can find it in M42 and Exakta mount, mine is in Exakta.
The Summarit 50/1.5 is interesting bokeh-wise. Not my interest but it was a good time trying it out. I prefer my Canon LTM 50/1.2 by a wide margin.