We use Hammer, which is bundled in Source SDK.
Open Steam -> Go to the Tools tab -> Download "Source SDK"
When downloaded and installed right click on it in steam and go to "Properties. then go to "Set launch options" and paste "engine -ep1" without quotes. Then open Source SDK.
At the lower section of the window are two drop down boxes. "Engine Version" and "Current Game" Choose "Half-Life 2: Episode One" and "Counter-Strike: Source" respectively.
Then under applications choose Hammer Editor and your ready to make some maps.
Peace.