Operation : Takedown, once a modification for Half-Life, has been resurrected for the upcoming Doom III by id Software. OpTD is a Single-Player modification that focuses on the intense action that counter-terrorist operatives experience during missions, and it will be a realistic mod, rather than a more action-based first-person shooter.
Currently, there are over 10 talented team members working on the many aspects of the mod. We aim to complete about 30-40% of the mod before Doom III is released. With its release comes the SDK, an extremely vital part of the mod's progress, and once we acquire it we can start the coding and mapping aspects.
At the moment, our website (http://www.optd.tk) has CS-themed background in two resolution sizes and a development picture of the Steyr AUG, modelled by Sklar. You can monitor the progress at the site, or join up to the forums and make contact with the OpTD Team personally to discuss anything mod-related!
Jeez... I just realised the typo... it should read "CT-themed background", not "CS-themed background". If you read the FAQ you'll see that OpTD is in no way a Counter-Strike ripoff. =)