I have an old, cheap aluminium frame (that carried me over my first few audaxes) knocking around that I am going to turn in to a single speed pub bike, to be quite literally used for going to the pub and general local errands. Basically something that has little value and I wouldn't be too upset about were it nicked.
I have a pair of rigida flyer wheels from a freecycle bike I built up for my wife. They need re-building but should be fine enough.
Originally I was going to go for a flip-flop hub but after some consideration realised that my needs would be perfectly well served by using the existing hub with a single speed conversion kit (spacers & sprocket).
The frame has a vertical dropout and I think I've worked out that 42 - 17 should give me about the right gearing (65") and a suitably taught chain. If that proves not to be the case I'll invest in a chain tensioner.
I've gone for a lowish gear as it's very hilly round my way. I'll be re-using an old chainset but just removing the outer ring, hence the 42T at the front.
I have most of the parts I need already knocking around the place. There's a few sundries I've had to buy but they've only totalled £40 which I pleased about in my mission to make the bike as valueless as possible!
If I ever come across an old steel frame with horizontal dropouts (or track ends) for little to no money I will likely get that and move the parts over, building up a flip flop at the same time.
Looking forward to the build and trying out single speed for the first time!