Originally Posted by wroomwroomoops

Building/converting a singlespeed, but have vertical dropouts and no chain tensioner? Use an old rear derailer/derailleur, it'll work great! Not only does it have really good springs and pulleys (generally much better than separately buyable chain tensioners) and is MUCH cheaper than a separate chain tensioner, even if bought new, but a derailer has the most comfortable way of adjusting the chainline - just adjust the H limit screw!

The only drawback is that a derailer is usually heavier than a chain tensioner, but you can always get rid or a few little parts on the derailer, to make it a bit lighter.