OpenSimulator is an open-source virtual world platform based off Second Life. Although I have not noticed any it is more prone to having problems than Second Life is since the way it is operated is less centralised than Second Life and some regions may not allow the user to use certain objects that they may have bought or created. Since it is open-source it is not developed by any one person or company but can be changed by anyone to meet their needs if they choose to.
It can be used for the same things that Second Life is used for and can potentially be less restrictive than Second Life as you are able to install your own copy of OpenSimulator or your own server or computer and not have to buy land and be subject to any terms of service.
There is an optional feature of OpenSimulator that can be chosen that allows an installation to be connected to the Hypergrid. The Hypergrid allows multiple OpenSimulator installations to be linked to allow avatars to be used in multiple different installations which was impossible on OpenSimulator before the creation of the Hypergrid as a new avatar had to be created on each new installation that a user wanted to visit.