The Nokia Lumia 520 is by far the most popular Windows Phone and also the cheapest, being available for as little as $29.99 unlocked. There are both good reasons why it would have been a good device to be blessed with the first wave of the Windows 10 Mobile Technical Preview, but unfortunately Microsoft did not include it.
It turns out that the XDA-Developers hack works as well on this handset as on the others, as demonstrated by this video by Hans Villegas, meaning if you have a disposable Lumia 520 you may be able to try running the new OS on it.
Note however Microsoft has warned of a high risk of bricking, so this should certainly only be reserved for actual devices you don’t mind losing.
Find the hack on XDA-Developers here.