Half-Life begot the mod Natural Selection, which begot standalone sequel Natural Selection 2, which together with NS1's Combat mode incestuously begot NS2's Combat mod, which now begets standalone game Natural Selection 2: Combat. It's all a bit like Chronicles with assault rifles.
Faultline Games are taking their Combat mod standalone, they've announced, expanding and polishing it up as one would expect. Combat's a murder-oriented version of Natural Selection, focusing on the asymmetric humans vs. aliens FPS combat while doing away with RTS-y bits like resource-gathering and commanders. It also splashes in RPG-y levelling, with murder-earned skill points unlocking new weapons, tech, life forms and whatnot.
I've never played the NS2 mod, but in NS1 would often hop on a Combat server to eat space marines while waiting for a slot to open up on my favourite 'Classic' mode servers. It was a fair bit more welcoming to new players than the base game, as the RTS layer can be almost as intimidating as the commanders yelling at you for ruining everything. Which also made it a welcome break from the stresses of Classic, or handy to hone specific skills. The standalone is unlikely to work similarly as a gateway, but then the whole idea is for it to be complete and enjoyable in itself.
"We've spent the last two months polishing and rewriting the parts of the mod we didn't like, adding new features and gameplay modes and starting to think about how we can make an interesting metagame," Faultine said in last week's announcement. No word yet on when it'll launch.