This is the most epic development I've seen by modders so far. (Other than Black Mesa: Source).
They've taken the best bits from Counter-Strike: 1.6 and Counter-Strike: Source.
It pretty much has the graphics of Source with the gameplay and basic map layouts of 1.6. The HUD looks pro, and it comes with some nifty little ingame options, like being able to choose the weapon angle from either 1.6 or Source.
It's free to download, the only requirement for it is to have a legit version of a Source Engine game on Steam.
http://www.cspromod.com/
(You don't need to sign up for anything, just click the Download tab at the top of the page under the banner.)