This is the 3D world most people have heard about. It’s been around for years and is one of the most “populated” worlds. There are at least hundreds of beautiful, exquisitely made regions to explore. There is also a developed in world economy with its own currency.
InWorldz is much newer than SL, but in many ways similar. The only difference is that there is a more relaxed atmosphere, for instance uploading files is free and you can find a lot of high quality freebies.
Craft World (Italian) is called ‘the friendly world’ and that’s definitely true. There are many high quality freebies and beautiful regions to explore. The people are as friendly as you’d expect from a friendly world.
Francogrid (French) is focused on education. There are some freebies and many interesting and well-made regions to explore. The people are friendly and though most of them are French, a non-French speaker can get by, either with a translator or because the French do speak English too.
Metropolis (German, Austrian, Swiss etc) is known as the ‘Free world’. Lately, many English-speakers have joined too, so it’s a multi-cultural world. There are some very fascinating regions to explore and some freebies too.
There are also several other 3D worlds, many of them based on opensim. If you have your own sim/grid and you have opened it for teleportation from other worlds, you can list it among all the other Hypergrid worlds.