Left 4 Dead 2 server status checker
Left 4 Dead 2 answers the standard Source query, but its lobby system means a reachable server is not automatically a joinable one.
Which ports a Left 4 Dead 2 server needs
Queries run over UDP on the game port. The complication is specific to this game: by default the server can be configured to accept connections only through Steam lobbies, which means a direct connection is refused even though the port is open and the query succeeds.
If it still shows offline
If this check passes but friends cannot connect directly, look at the lobby-only setting rather than at your router. Disabling it allows direct connections at the cost of the matchmaking integration.