While there are a lot of anti-union companies, and there are labor disputes that crop up and then get resolved (most of the time), there are some companies that are downright bad enough to boycott.  Below are links to the official union boycott lists. 

AFL-CIO Boycott List
http://www.unionlabel.org/index.cfm?zone=/unionactive/view_page.cfm&page=Boycott

Hotels to Boycott and watch for labor disputes
http://www.hotelworkersrising.org/HotelGuide/boycott_list.php