A lot of people have created simple games using processing. It isn't a game library, but having the basic graphics loop and mouse/keyboard event capturing is a great start. You just need to start programming your own rules for how things behave within the loop and how they respond to user input.

Zach Gage and I made a version of space invaders last year using processing.
http://vimeo.com/912985

I've been comparing Processing against a couple of 2D gaming engines. I reimplemented the Gosu Ruby tutorial http://code.google.com/p/gosu/wiki/RubyTutorial using the Love engine http://love2d.org/ (in Lua ) and Processing.

It is more a demo than a game; you have a little spaceship that you drive around the screen and collide with stars.  The Love version has exploding stars using Love's particle system.  

I needed to implement a rotatable 2D sprite class http://processing.org/discourse/yabb_beta/YaBB.cgi?board=Syntax;action=display;num=1222098638 to handle the spaceship and a refresh-rate smoothed animation class for the spinning stars.

Processing's sound libraries have sophisticated features like FFT analysis but don't handle looping background sounds very well.  Minim has a big audio pause between loops.  Ess causes draw() to stutter when you stop or pause a sound.  In the end, I used Ess and set the volume to zero to avoid the stutter.  I also had to play with the format of the sound files, Ess fails to load some combinations of codec/bitrate.

Processing stacks up pretty well.  Most of the coding was straight forward and any complaints I have are about Java, not Processing.  The sketch comes at just under 250 lines of code and runs nicely in the browser.  

I'd be happy to send you the sketch if you give me your email address in a private message.

Love is interesting, I think I had passed over it before because it hadn't yet gained an OSX version.

I started learning processing to try to get my feet wet in object oriented programming in the hopes of developing a 2d game. I'm finding that I'd really not like to reinvent the wheel for some of the stuff I need though- Sprite classes with collision gravity and some other basic physics, scrolling, the notion of a 'player'. And I'm wondering if there's a more game oriented environment out there for me that isn't just a 'game construction kit' that makes the same game over and over but with different graphics.

I'm not giving up my processing studies, but I might give Love a try if there's a user group already sharing those sorts of code bits.

cool !

Thank you, Quark!

That's awesome