cert-quicklook is a plug-in for QuickLook to easily generate previews of various unprotected certificate tokens.
Currently includes support for X509 certificates, DER or PEM (ASCII-armored).
Future plans includes extending for other types of data: keys, revocation lists, signing requests, e.t.c.
Tested with MacOS 10.5.8/10.6.6/10.7.1
More information can be found here: http://code.google.com/p/cert-quicklook/