Abilities ?

[xDeGroatx]

If i have an artifact that taps a creature on the feild and my opponent taps his creature to attack, can i tap that same creature in response?

[Remillo]

The latest you can tap down a creature to prevent it from attacking is during the Beginning of Combat Step.  However, most people will simply skip over it and declare they're attacking you with some creatures.  You can usually say "Hold on, back during Beginning of Combat" to back up and do what you intend.  However, if you opponent states "Moving to Combat", then gives you a chance to respond before saying "Declare attackers?", that's the window you have.  If they shortcut to attacks and they attack with five creatures, and you have them back up to tap one down, they're also not bound to attack with the other creatures, since something changed and the shortcut was declined.