Tlen.pl is an adware licensed Polish instant messaging service. It is fully compatible with Gadu-Gadu instant messenger.The communication protocol is based on open source jabberd code, but it was modified significantly, making it incompatible with generic XMPP clients and servers. Among other features, Tlen.pl allows for voice chats, SMS sending and video conferences. The versions later than 4.0 allow for creation and usage of plugins.