Writers Weekly
http://www.writersweekly.com/markets_and_jobs.php
They have two useful sites for freelancers, a list of paying markets and a list of jobs.

Absolute Write
http://www.absolutewrite.com/freelance_writing/markets_online.htm
Large list of links to publications that purchase articles.

Sun Oasis
http://www.sunoasis.com/sunoasismarkets.html
Freelance markets

Writing for Dollars
http://www.writingfordollars.com/Guidelines.cfm
Searchable database of publications that buy articles.

Writers Digest
http://www.writersdigest.com/guidelines.asp
This is a free sample of their larger subscription-based database.