halflife-sdk
There are 3 repositories under halflife-sdk topic.
FreeSlave/halflife-featureful
Half-Life SDK for making mods upon it
Xaymar/hl1-mastersword
Master Sword is an RPG mod for Half Life 1 (GoldSource)
SanyaSho/studiomdl
A cross-platform compatible version of studiomdl for Half-Life.