The transits of Mercury in front of the Sun are more difficult to observe but occur more often that the Venus transits. 
 

• What has occurred on May 7, 2003 (prediction of the event - in french) 

• A site to watch the observations of the transit of Mercury of May 7, 2003 (Paris observatory) 

• The SOHO site for the observation of the Mercury transit from space

• The predictions of the transits of Mercury from -2999 to 3000 (in french).

• The web site of USNO