I've played Roblox, Animal Jam, and Minecraft, but the most recent game i have played is Pirate101.
Roblox is a popular website full of user created games, the downside to this is that the games are multiplayer, and people have found loopholes to the filter, among with many other reasons. Tons of people have migrated to Minecraft because of the criticism it has went through.
Animal Jam is a kid's mmorpg, and it is all about creating animals and buying items for their animals, etc. but it was originally meant to be just an online game. The change was caused by players deciding to roleplay characters based on the book Warriors. But some people just roleplay as animals themselves. Most of the roleplaying occurs on the server Aldan. Learn more about this online game by visiting their website, and don't forget to create an account!
Minecraft is an indie game, and it has two separate modes, creative and survival. You have the decision to play multiplayer servers, but you'll have to find a server before getting started. Survival focuses on the crafting interface, getting food and shelter for your avatar, and surviving the night without getting caught by the monsters that lurk in the darkness. Creative focuses on building stuff, limited resources, and flying!
I will compare these games to Pirate101 in the comments, and be sure to compare some of the games you've played too!
Re: Comparing Pirate101 to other games i've played.
I hate Roblox because it's broken two of my computers, I kind of like Minecraft, but feel like it's a bit over-rated, and Animal Jam? I played it for about a week before a) getting bored, and b) people asking me to date them in-game. No. No. No way. It's a kids game about little animals and mini-games on National Geographic! NO.