I don't even remember what I released... its been so long..
Well, they usually end up being my breeding experiments. Which have been, Cleffa, Hawlucha, Pineco, Bulbasaur, Sandile, Mawile, and Growlithe. I usually end up having tons of these guys, and rather than deleting them, I'd rather send them out into the wild as it were to see if they find a home somewhere amidst the Wondertrade system. I didn't start wonder trading till well after completing the game, so I never got to experience the joys of trading a caterpie and getting a Xerneas for it.. for better or worse.
What I get in return, however, is usually the stuff I release, resend out or keep depending on what it is, as you probably would of guessed. I sometimes end up with good stuff, Spewpa, for example, is something I always keep because you can evolve it and trade it for just about anything if you happen to get one from a region thats not all that common. I usually keep fossils, because they tend to also be "hot items" on the GTS from what I've experienced. Though they haven't netted me a Heatran yet, so I guess anyone who has one probably won't let it go for anything less than a Keldeo or something silly like that... I usually always resend out a Pokerus guy though, being I have several with it already, and deleting it would be a waste.
Moving forward though, I really hope Gamefreak overhauls the way you store Pokemon. The numbers be climbin, and it's just so tedious and kind of backwards doing things the old way.