I can't find my server listed in the Server Browser, what's wrong?
The first thing to check is to make sure your firewall and router has port 27015 open and forwarded (or whatever port you are using for your server). You have to have both UDP and TCP forwarded for the server to be listed and accessible.
If this is a listenserver, you need to make sure you have sv_LAN 0 set in console or in your launch properties or in your listenserver.cfg file. The typical error seen if that is set wrong is LAN Games Restricted to Class C Clients error.
If you are behind a router that is doing NAT translation, your Server will have a different internal IP address than what is needed to list in the Master List (players can't connect using your internal IP address).
You can also add some additional region information to make it easier to find your server: sv_region #
If you set sv_region 255 then your server is only listed in world but if you set one of the other regions then your server gets listed twice, in the region you set and in the world. If you set an invalid region things will be unpredictable.
Here are some additional reported symptoms that are often reported as Server listing problems but are client related (not Server related):
If you only have a problem with servers listing or staying listed or in history (but can play otherwise (client problem), you should be able to solve the problem by closing Steam, deleting masterservers.vdf and serverbrowser.vdf in the Config directory, and re-launching Steam.
You also might be able to clear up the problem by changing your Steam skin to the default, or to gray if you're already using the default.
Otherwise the source of the problem is probably your router. If you don't have a router, it could be because QoS or uPnP is enabled on your server, because you are on a wireless connection and XP "Wireless Zero" service is enabled, or because you have some other software installed that's interfering with Steam.
If you have Norton Internet Security installed, try temporarily disabling this service (don't kill the task, use the normal Norton Services exit) to see if that clears the problem up.
Likewise ZoneAlarm Pro and most other firewall programs can cause problems if they're not properly configured. Windows XP users, please note: the XP SP1 firewall will not cause these problems, but the XP SP2 firewall might if it's not properly configured. You can search this Knowledge base for more help on configuring your hardware or software firewall.
If you do have a router and it's separate from your modem (cable or DSL), try bypassing your router and plug your computer directly into the modem. If it's your router, the problem should then clear up.
If you've identified it as a router-related problem, it could be because SPI-mode or uPnP is enabled on the router. The problem with SPI-mode has been reported for the D-Link DI-604, with firmware 1.81 (Euro).
Other router issues are related to the firmware installed. Problems have been reported with routers and modems from Gigafast, Linksys, and Actiontec, and in some cases you'll need to downgrade your firmware to an older version in order to clear up the problem.
Please contact the tech support for the manufacturer of your router to get assistance.