An indispensible resource.  Hands down the most comprehensive -- and user friendly -- site for rebutting deniers.  Creator John Cook has identified the 165 (and counting!) most common denier arguments, and solicited  definitive rebuttals from climate scientists.

According to author Coby Black, this is "a comprehesive set of rebuttals to common 'climate skeptic' talking points vetted and endorsed by the professionals at Real Climate." 

Think of this site as a useful complement to Skeptical Science (SkS).  If you can't find what you are looking for in SkS, or want additional  ideas, look here.

Real Climate is a very reliable blog by active climate scientists, so if they give it the "thumbs up" you can rely on what your read here.

Graphic artist Peter Sinclair rebuts climate skeptical arguments with short, snappy videos that manage to combine humor and lush graphics with solid sourcing in the scientific literature.  

Sinclair narrates -- calmly and quietly, never preachy or pedantic -- but lets climate experts themselves make the key points.   He often employs a very effective technique of visual  documentation:  First he shows an actual photo of the most relevant peer-reviewed journal article, then he zooms in, enlarging a single key sentence that  punctures the denialist argument.    You'll feel like cheering!