Valve Hammer Editor (sometimes shortened to "VHE" or "Hammer") is the official editor for
GoldSrc and
Source mapping. It was previously known as Worldcraft. The most recent version for GoldSource is 3.5. Hammer is also included in the Source SDK which can be downloaded via Steam (Tools > Source SDK, provided you have any Source games).
While Hammer is the official editor, there are some alternatives that provide greater capabilities, each with their own pros and cons. Some alternatives to VHE for GoldSrc mapping are the
Sledge Editor and
J.A.C.K.. For more information, see the
Valve Developer Community wiki.