A few links:

• The official site of Putnam competition
• Homepage of the organizer, MAA (Mathematical Association of America)
• Putnam problem archive (1985-present)
• Lots of useful links here
• A complete Putnam archive
• Putnam training sessions at U Waterloo (historically they did very well).
• Here are a few tutorials I wrote:
  • On generating functions
  • On recursion
  • On algebra
  • On analysis
• Handouts: 2011.10.13