The following ports must be open in your firewall and router for Steam and Steam Games to work:

UDP 1200 (used for friends service)
UDP 27000 to 27015 inclusive
TCP 27020 to 27039 inclusive
TCP 27040 and 27041 only for CyberCafe Owners

Computers running Dedicated Servers need these ports open:

UDP: 27015 and 27020 (default HLDS, SRCDS and HLTV port)
TCP: 27015 (SRCDS Rcon port)

If your server uses a different hostport then you will need to adjust the above accordingly.

For help on port forwarding for your Router, please consult this excellent website: http://www.portforward.com.

For assistance with your Firewall you will need to go to the support site for your particular Firewall.

NOTE: Windows XP SP1 firewall does not require any specific configuration changes for Steam Clients to work, but for Source Dedicated Servers you must open TCP Port 27015 (or whichever port your server is using) for RCON to work!