Spirit is a mod that gives mappers a whole new set of entity options and extended possibilities. Not really a tool used 'in Hammer', but a mod. That means it's playable, though it doesn't give you any more than a few example maps of its capabilities. And that's the point: it lets mappers do more things than they could do with plain HL.
I wouldn't recommend Spirit if you're a mapper just getting started, or still feel uncomfortable with HL's entity set, though.