RangerWickett said:
Bah, don't listen to Nightfall. He's not telling you the very important fact that 'Scarred Lands' sounds too grim and gritty to interest me, and of course, only my opinion counts.
One of these days I'm going to have to ask him why he loves the setting so much, and why he first got into it.
Nightfall loves collecting errata and broken rules so that's why he loves his Scarred Lands collection.
Going back on topic, Felon, I would actually suggest FR unless the players are really insistent. The world is big enough to still run a cool campaign without it being ruined by the next novel that is published. As much as I dislike the fact that the novels too often drive the game products, the novels do serve a useful purpose in providing a quick intro to the setting.
Kalamar would also be a really good choice as the product support levels are excellent plus there are no novels to "wreck" your world. As Mark from Kenzer mentioned above, the website includes some good intro material. One warning, though: some of Kenzer's rules are pretty broken and require re-design or re-editing.
I wouldn't recommend Scarred Lands because, my joking comment aside and speaking from purely personal opinion, there were enough rules and editing errors in their first few publications for me to never to consider purchasing their products again.
Greyhawk is also very good and there is a stack of stuff available on the 'net that would provide your players with a really good introduction.
A more off the wall choice would be Oathbound. The players are transported to another world which might be a good idea considering the way your campaign has developed. A big positive is that you really/genuinely only need the setting book to play there. There is enough material in that to run multiple campaigns.
Normally speaking, I would also strongly recommend both Midnight and Dawnforge but neither are suitable for simply swapping existing characters into. However, for your next campaign....

Check out
www.againsttheshadow.org and
www.pathsoflegend.org to see if your appetite could possibly be whetted....
Cheers
D