DShield brings us a daily update of the Top ports attacked:

http://www.dshield.org/topports.html

And again from SANS, a GREAT document listing common security holes in ALL systems (they also list open ports near the end of the document):

http://www.sans.org/top20.htm

We got attacked today on port 21, (FTP) but luckily they did not break through (at least I hope so!). They were masking their IP address, we couldn't track their number down.