Adapations of boardgames to different platforms (such as computers and mobile devices), as well as to new rules or specifications outside those provided by the designer (e.g. number of players outside the 'official' range).
Adaptations of boardgames allow people to play a game in different circumstances than originally envisioned by the designer of the game.
This includes adapting the boardgame to run on a computer or with computer assistence, run it on a mobile device or on some game console.
Other adaptations occur when the game is modified to suit a different number of players.
The actual pieces of the boardgame can be adapted as well. Ranging from making the board to play on more robust/stiff to complete redesign of the pieces.
New rules seems to be the most available adaptation. This happens accidently when people misread the rule book; start mixing different versions of the rules or come up with complete new ways of handling events in the game.