What are some strategies that use the Raven in Starcraft 2?

There are several good ways to make use of the raven, eventhough it's in every case just a supporting unit.

The easiest one is raven harrass, simply move into the back of the enemy base and put a few auto turrets in his worker line or fire a seeker missile.

For bigger battles, the point defense drone is very effective against most ranged units, allowing your units to survive longer and even overcome an enemy with a slight advantage. It won't turn a battle on it's own though. The seeker missile is imho not that effective in a big battle since the targets it would be very effective against (zerglings, marines) die very fast anyways.

The biggest use for the raven is against banshees, dark templars or to find burrowed units or as spotter for siege tanks (though it's too vulnerable and expensive for that usually).

One thing I occasionally do with them is drop 3-4 auto-turrets on an unsuspecting expansion and take out a good amount of workers. Unless they have a fair amount of troops already there you'll take out some workers and it'll just cost energy.

I mainly use them as mobile detectors. I usually buy a quick raven early to do some quick harassing of my opponents mineral line, and then I ususally keep a handful of them around for the rest of the game for detection purposes (cloaked things easily get -so- out of hand, and by having a raven I can save my scans for more important things and I also get a caster with several good abilities that can help me in other ways).

So imho, ravens are just a support unit. I don't really see the use of having many of them around, but I always tend to get at least 2-3 of them in almost every game I play.