Start with any number of counters in any number of piles. Two players take turns to remove any number of counters from a single pile. The player who takes the last counter is the winner.
Read the Nrich article – Meet the Nim Family which includes Got It, an interactive version of the classic game of Nim and also discusses the benefits of using games. See also Nim-like Games and from wildmaths.org try Fun with Drips.
From AMSP their Online Enrichment Event recordings include Race to the target! which introduces students to the family of games of NIM and looks at strategies to win in different situations. A recording and teacher notes are provided. The teacher notes which include questions for students to think about are really helpful.
From Jefferson Lab (Thomas Jefferson National Accelerator Facility – Office of Science Education) this game of Nim allows you to play a friend or the computer. If you choose the computer to play against you can choose whether the computer should be an expert, somewhat distracted or mostly clueless!
How to play Nim – Cornell University.
From a BBC, see this article on Nim Strategy.