Here's what Wikipedia has to say about Contract Research Organizations (CROs):
A Contract Research Organization, also called a Clinical Research Organization, (CRO) is a service organization that provides support and/or services for clinical trials. CROs support the pharmaceutical, biotech and medical device industries as well as foundations, research institutions, and universities, in addition to governmental organizations (such as the NIH, EMEA, etc.). There are over 1,100 CROs in the world. It is a very fragmented industry with the top 10 controlling 56.1% of the market in 2008. One estimate had the size of the market set to reach $24 billion in 2010 and set to grow at a rate of 8.5% through 2015.
Top 10 CROs:
1. Quintiles ($2.5 billion)
2. Covance ($1.8 billion)
3. Pharmaceutical Product Development (PPD) ($1.4 billion in sales)
4. Charles River Laboratories (CRL) ($1.2 Billion sales)
5. Parexel ($930 million in sales)
6. ICON ($887 million)
7. Kendle ($590 million in sales) 
8. Pharmanet ($470 million in sales)
9. PRA International ($410 million in sales)
10. Aptuit ($370 million in 2009)
Next 30 largest by marketshare (2008 figures)
(sorted alphabetically, i.e. not in order by marketshare)
* Bio Imaging
* Chiltern ($111 million in revenue 2009)
* EGeen International Corporation
* Eminent Research Systems, Inc.
* Galapagos (Biofocus)
* GVK Biosciences
* INC Research
* LAB Research
* Life Sciences Research
* NDA Regulatory Science
* Novella Clinical
* Paragon Biomedical, Inc.
* Pharm-Olam International
* Radiant Research
* SNBL USA
* Stat-Trade Inc.
* Target Health Inc.
* WuXi Pharmatech Co.
References are on the Wikipedia page here.