I have found four main ways to find your steam ID; and here they are;

[1] - From Ingame
When you are ingame, you can press the ¬ button on your keyboard to bring up the console, if your console is not enabled navigate to options, under the Keyboard Tab "Advanced" button. There should be a check box for "Enable Developer Console".

Once you have your consoel up, on any server, type "status" you will then be able to copy / paste your steamid.

[2] - Use SteamID Finder
This program allows players of valves online games like Counter-strike source / Day of defeat Source or any other Source engine based game to find their unique identification ( Steam ID for short). It finds your steamid by scanning the Steam logs
Download here

[3] - Search Online Stats
if you have played on our servers search through our stats all steamID's are with player information, if you havent played on our servers try searching for yourself on the HLStats Global ranking's

[4] - Search Local Logs
Locate your steam directory on your hard drive, usually C:\Program Files\Steam -- find steam.log file and open it with notepad. Look for lines such as: Found existing connection 9,216.228.115.67:27030 for 0:0:8888. The last few numbers is your steam id, ie: 0:0:8888.