Recently we have revived a habit of playing lots of 2-player Settlers of Catan. The official rules don’t have a 2-player version, but it turns out that just using the ordinary rules as is gives a tolerably good game.
The most significant problems with the game played like this are: essentially no trading between players; possible for one player to be plagued by the robber for long periods of time. I do not propose a solution to the trading problem.
There are lots of 2-player rule variants available on the internet which try and help the robber problem and other problems. Of the ones we’ve tried (and invented), none seemed simple enough.
Then we had the following idea, which we have tried a few times now and seems to work well:
The robber can only rob for 4 rolls of the dice. In other words if the robber is on a hex (other than the desert) and hasn’t moved for 4 rolls, then after the 4th roll it is returned to the desert hex.
The other value of 4 that we have tried is 3, but I think 4 seems to work slightly better.
We use a coin to maintain the count (you’d think you’d be able to count to 4, but it’s surprisingly tricky). The coin is given, heads-up, to the player who has roll number 2 (that is, the player whose roll is not next). After you roll if you have the coin then you either flip it over, if it was showing heads to begin with; or, return it and the robber to the desert, if it was showing tails.
All the other problems with the 2 player game seem minor in comparison and can be compensated for by playing more games in a row. The 2 player game is quite short.
[Much later: 2009-11-22: Making it not quite so simple, we have added a further rule (to the 2-player game): Longest Road and Largest Army are worth 1 point each; they're roughly twice as easy to get and keep compared to the 4-player game. The "4 turns" rule for the robber continues to work quite well.]