I made a thread on freezing alcoholic drinks to increase the percentage of ethanol a while ago and since then I've found out that its pretty effective. I haven't tried it with spirits I've tried it with beer and red wine and in a matter of hours in the freezer ice forms in the solution and obviously that ice isn't solid ethanol.
Can you list OTC substances which could be extracted or concentrated using freeze concentration you can think of off the top of your head. The first thing that comes to my mind is vinegar. Acetic acid has a higher solidification point that water so it would be the filtrate we'd be after here. Are there any complications I'm not taking into consideration here or could you concentrate acetic acid by cooling white vinegar?