Minecraft prison servers have cropped up all over the net lately. It's a not so new game mode in Minecraft that forces you to work your way through levels letting you earn your way all the way to freedom. In a way it's similar to older games where you're single mindedly trying to accomplish your goal, but you have open hands in how to do it. There are plenty of ways to go about it. Some people prefer to mine as quickly as possible to get to higher levels where population is less dense so there's more of the valuable materials available, however another strategy is also to mine some of the rarer materials and then trade with your fellow players, letting them get the gear they want to make their journey faster and in exchange you get the money to level up, but this strategy works better in more populated levels since obviously the more customers you have the faster you're going to be making money.
Some servers also started spicing up the prison gameplay by running events, jumbling up the prices of certain materials, which lends credence to those who hoard certain blocks, letting them sell them for a huge profit while others languish in lower prices. There's is obviously also the other method of simply killing other players for their loot before they manage to sell it. This method might seem a bit unseemly but it's a valid strategy and usually works better in groups. If you're looking for a server list of Minecraft prison servers, you should be able to find plenty of them on several of Minecraft server lists floating around the internet. I can honestly recommend this Minecraft server list, however there are plenty of others so don't be afraid to shop around a bit.