Before last season, Tom Tango (co-author of "The Book") posted a comment on this site about the Clutch Hitting Project.  Essentially he was recruiting fans to help with a test of clutch hitting.  He invited fans to submit hitters on their team who they perceive as good in the clutch.  Tango prepared one pool of players  from each team which he selected, and used that to test against the fans'  clutch hitters.  He reduced the fans' pool to take out players who were in his comparison pool.  I submitted two Astros' hitters, but I can't remember whom.  I may be wrong, but I think perhaps Loretta and Tejada.  From the Astros, Tango ended up with Wigginton in the good overall group and Erstad in the clutch group.  How Tango picks the groups is complicated, with references to the Nash Equilibrium and "a beautiful mind."

He wrote about the results here.  The basics are that the fans selected a group which was more clutch than the comparison group.  But the impact is modest in size.  The fans' group weren't better hitters in the clutch than Tango's pool (the comparison group is supposed to be good overall hitters), but they increased their performance more in the clutch.  He discusses the margin of clutchness:

Is that a big deal? Well, it's less than the platoon advantage, which is 20 wOBA points. So, when you give consideration to wanting a clutch hitter at-bat, you have to temper your enthusiasm with the understanding that that clutch skill is less than if you had a similar batter with the platoon advantage. No one is going to select Marco Scutaro over Alex Rodriguez. The two players must be pretty close to begin with in talent, before you go off having a preference for your clutch hitter over someone who is otherwise a better hitter.

The project also comes up with some interesting findings about players who fans perceive to be clutch.  In general they tended to be contact hitters, who hit more singles and less HRs, and strike out less.  Tango notes:

The guys they selected as clutch put the ball in play (excludes HR) 76 percent of the time, compared to my great hitters of 67 percent, in all situations. Those numbers dropped 2 percent points for both groups in clutch situations. The selection criteria by the fans on this basis was nine standard deviations from the mean, showing a fantastically clear bias in this regard. It’s very possible that to a fan, clutch is all about doing what Carlos Beltran didn’t do in his last at-bat against the Cards, when he took strike three.

That is an interesting observation.  I think a fan bias does exist for players who make contact instead of trying to draw a walk in clutch situations.  Recall how vehemently people reacted to a high OBP hitter like Morgan Ensberg when he would take a called strike three with men on base.

I also think that fans are biased in favor of hitters who can get  a runner in from third base with a single or sac fly, as opposed to hitters who swing hard and sometimes miss (but sometimes hit HRs).  Hitters who don't get the runner in from 3d base with less than 2 out are very frustrating to fans, and that may stick in their minds.  Carlos Lee's ability to get the sac fly in that situation is one of the reasons fans like him in clutch situations.