While trying to resolve a ports issue on a staging server, I happened to run an nmap on our production server, and found that port 800 was open:
800/tcp open mdbs_daemon
I have no idea what mdbs_daemon is - I have googled the port number and the daemon, but can only find a discussion hinting that it is something to do with NFS, but I'm pretty sure I've not installed anything related to that on my Ubuntu 10.04 production server.
I'm just not sure how concerned I should be at this unexpected port.
If you have a proxy server installed this could be a port used by squid. But it really could be anything.
Use this command to try to identify the service, not just guess it by port number, which is the default behavior of nmap :
You can also use this to see the program "linked" to the port :