Vanilla Minecraft servers return a full lit of online players, no mods needed. I would say anyone that plays the game is already opt-ed in; if they don't like it, complain to Mojang.
Unfortunately unless the server uses
RCON/
Query, it's nearly impossible to track users easily. The
0xFE packet that the server sends to clients' server lists only generates a player count, not a list of players. Although, nothing is stopping going further and getting the
0xC9 packet from a server, it just takes a lot more work since you have to login to the server first which involves getting a token from minecraft.net which also means passing a username/password verification.
I'm just not sure which one is harder to do in terms of quick implementations. If ryanteck is interested, I can hand over my PHP document that shows player count and server description, but it's not much of use to him I'm sure.