If you bought the game, yes it is free from there on out. I guess you are looking for a Minecraft Server that is free to play on right?
In that case, just browse through the many thousand of servers I have on my list at http://minecraft-server-list.com/ or any of the other Server Lists out there. Those are the easiest places to browse through a lot of servers that is online right now.
I would just recommend one of the smaller servers to start out on - find a few friends ingame and make stuff together. The Big paid servers you must be thinking of is for the players that seek some special kind of gameplay or special rights to do stuff in-game. I would not worry about them at all.
Enjoy the game :)
And if anyone comes here, wanting to play Minecraft for free (without buying the game first at http://www.minecraft.net/) - well, that is not allowed as I cannot help in that. This game is MORE than worth the cost (counting the hours you will use on the game).