Here's a list of 3rd party grading services for comic books. I'll be updating this list as new companies are formed.
CGC - Comics Guarantee Company LLC
The biggest and most popular third party grading company.
CBCS - Comic Book Certification Service
The newest and second most popular comic book grading company.
Below is a list of grading companies that I have not reviewed. Some will be reviewed in the future. Use these companies at your own risk.
EGS - European Grading Service
PGX - Professional Grading Experts
60 Day Comic Grading
MCG - Midwest Comic Grading
CBGC - Comic Book Grading Company