There are quite a few of them out there. The FERET database (http://www.itl.nist.gov/iad/humanid/feret/feret_master.html) is one of the largest ones, with thousands of photos.
Some others which may be of interest:
U Mass's Labeled Faces in the Wild: http://vis-www.cs.umass.edu/lfw/
Yale Face Database: http://vision.ucsd.edu/content/yale-face-database
ATT's Olivetti Database (linked above by Luca Pinello)
If these don't suit your needs, I recommend scouring recent web pages on the type of facial recognition you're doing; many of them will have links to image databases with the characteristics relevant to them (grayscale vs. color, frontal vs. various perspectives, lighting variations, posed vs. in the wild, etc.).