For statistics on M&A activity
http://www.mergerstat.com/quick_charts/index.html
For news on M&A announcements
http://biz.yahoo.com/me
Legal Primer
http://profs.lp.findlaw.com/mergers/
Federal Trade Commission - Mergers and Acquisitions
http://www.ftc.gov/bc/bcmergacq.htm
Online brokers
http://www.business-buyer-directory.com
http://www.emergers.com/
http://www.firstlist.com/