Valve Hammer Editor -
Version 3.4,
Version 3.5 beta. The editor of choice for Half-Life mapping and also the map of choice for
XashXT modding as well. An alternative is
Jackhammer, created by XaeroX for Chain Studios, which has the same interface as Hammer, only improved and fixed. Take note that XashXT is not fully compatible with Jackhammer and such is used at your own risk, otherwise, use the Hammer Editor instead.