The union that's organizing Wal-Mart stores in Canada has a site for Wal-Mart workers to learn more:
http://www.walmartworkerscanada.com/
ILRF is organizing campaigns to change Wal-Mart's supply chain practices, including spearheading a legal action by Wal-Mart supply chain workers claiming massive, systematic hours and wage violations.
http://www.laborrights.org/projects/corporate/walmart/index.html
There are two major US-based campaigns trying to change Wal-Mart's practices on a whole range of issues. Find out more at:
Wake Up Wal-Mart: http://www.wakeupwalmart.com
Wal-Mart Watch: http://walmartwatch.com/
Similar to the US campaigns, a website dedicated to ASDA, Wal-Mart's UK subsidiary
http://www.asdawatch.org/
Also, check out the Alternative Report on ASDA Wal-Mart, available at:
http://www.waronwant.org/download.php?id=369