Advertising image albums - Not checklists, but many great images of albums and cards
Football Cartophilic Info Exchange - Focuses on football (soccer) cards with many great checklists
Jeff Allender's House of Checklists - Thousands of checklists for cards of every type
The Movie Card Website - Dedicated to movie star trading cards
Pre-War Cards - Blog and checklists for pre-1948 sports cards
Non-T&F checklists - these checklists are for series which don't include any track and field cards, but for which I couldn't find complete (or nearly complete) checklists anywhere else.