Playing pool online for free can be a whole lot of fun. Originally you could only play pool online in smaller flash based games where you were the only player however now there are more and more pool games launching where you can play against other people (not just computers) without having to front-up with cash for the software.
Why are the pool games changing and which style of game should you be playing?
The pool games are changing as more networks that run play for cash pool games are launching. A few years ago, you couldn't play pool for cash, you could only play it for free and the games were always part of a bigger flash games website with a whole lot of other flash games on the site as well. Now, with networks launching who are planning on offering cash games, they are interested in gaining players who play pool online for free and trying to get them to move over to play for cash.
Which game should you be playing? That's really up to you.
In the end the real decision should come down to whether or not you are interested in pool online for cash or for free. If you're more into it for a bit of a laugh and killing some time online at work then flash based games sites where you can play a lot of pool online as well as other flash games is probably the right style of pool game for you.