After the "SteamPipe" update of Half-Life, Valve released the SDK on Github, for the most up-to-date licence you should check the official repository
. The license from Github is below.
*Note*: This is retained for archival purposes, but this EULA is out of date and only applies to the old installer-based SDK. The updated license actually seems slightly less restrictive (this older license specifies distribution rights "solely to licensed end users of Half-Life", the new one does not specify this), so there's no real reason to use this old license any more.
Below is a full reproduction of the EULA presented in the Half-Life SDK installer. It is included here as reference material for documents reproduced from the SDK.