>>717080676
HL1 except you're a regular scientist and you have to craft basic weapons and gear to survive the monsters that keep popping in. The crafting isn't grindy though. You also use the portal technology that your lab is working to visit various other worlds.
There's more plot than HL1 and it's more complex. More named NPCs.
Large maps, varied enemies, good difficulty
Can be played solo or co-op