Rule changes, additions and options proposed by players for players. This is not for officially sanctioned variations, see below!
If there is anything boardgamers like better than playing games, it might be thinking about ways to change or tweak them to suit our particular taste.
Discussion about ways to vary a boardgame are definitely on topic here. If the variation originates with an official source (the publisher or designer) then the question should be tagged with variants.
As with most other subjects, you will get the best answers if you make the question specific. For house rules, this typically means giving a specific goal that you would want to achieve with a house rules. Thus, "How can I prevent [boring yet effective strategy x in game y] from winning by adopting a house rule?" is better than "How do I make [game y] more fun?".