All things the squadmaker does in svencoop.
and more..
Not really sure on how to implement SC fgd and such to a SP HL map. But, if you want lots and lots of neat features for mission type maps then SC expand on the original HL tenfold. And you can play the map with one or more friends if you like.
Captain is right though, the monstermaker is better used to spawn hoards of monsters, or items. Mostly because you dont want a million monster entitites in the map from start, and you might have limited space to work on. If you want control in HL then the porting in from other place option is far better.
Still:
www.svencoop.com
I see no reason making a singleplayer map when you can make it coop, but that is of course a matter of opinion.
Yours
Umbrood