5900 tcp vnc
VNC (Virtual Network Computing) - remote control program, http://www.realvnc.com/
It also uses ports 5800+ and 5900+ for additional machines.
Backdoor.Evivinc also uses this port.
Some Apple applications use this port as well: Apple Remote Desktop 2.0 or later (Observe/Control feature), Screen Sharing (Mac OS X 10.5 or later)
RealVNC 4.0 and earlier allows remote attackers to cause a denial of service (crash) via a large number of connections to port 5900.
References: [CVE-2004-1750], [BID-11048]
W32.Gangbot (2007.01.22) - a worm that opens a back door and connects to an IRC server. It spreads by searching for vulnerable SQL servers and by sending an HTML link to available contacts on instant messenger programs. It also spreads by exploiting the Microsoft Internet Explorer Vector Markup Language Buffer Overflow Vulnerability [BID-20096] and RealVNC Remote Authentication Bypass Vulnerability [BID-17978].
Vino 2.28, 2.32, 3.4.2, and earlier allows remote attackers to read clipboard activity by listening on TCP port 5900.
References: [CVE-2012-4429]
Remote Framebuffer (TCP/UDP) [RFC6143] (IANA official)