House Rules generally used at GamesEvening to Catan games:
Allow trades and builds in any order on your turn. An official rule in Starfarers and I believe CitiesAndKnightsOfCatan; a very common house rule in SettlersOfCatan.
Trading with the bank through ports (or equivalent effects) permitted at the end of *any* turn - or, in fact, in the middle of anyone's turn. (Applies to the /CardGame and Starfarers also.) E.g.: the player whose turn it is says to the player with the ore port, "If I give you 2 ore and a wood, will you trade the 2 ore through your port to make a brick which you give back to me, and you keep the wood?"
If a player gains no resources after a roll (and this is not due to a robber or a 7), that player receives a counter. Counters may be traded in for one resource card at N:1, where N is 4 or the player's current number of victory points if they have more than 4. If a player has the Aqueduct in Cities & Knights, this no longer applies as they're getting resources from those rolls now. Also not used in Starfarers, because the Reserve Deck was introduced to deal with precisely this issue.
Production chits (2 through 12), or the hexes of the board, may be rearranged, before 1st player is determined, and by unanimous agreement. Generally this is done to ensure a more balanced board. Particularly important in Cities & Knights, due to the way one can't simply wait around for the scarce resources to turn up. It's crushing when the barbarians attack early and there physically hasn't been enough ore, sheep or corn to get a defense force active: it makes the outcome more luck-based and is thus generally bad.
Initial road positioning is fluid: generally players are allowed to reposition the roads from their initial settlements in response to other players' settlement or road placements.
Robber not used at all. A roll of 7 forces discard for anyone exceeding hand size limit, but then the current player chooses to either steal 1 card from anyone's hand, or to re-roll to get a non-7 production roll. Probably less necessary in /SettlersOfTheStoneAge as all players can reliably buy a move of the robber.
Optional addition to the above: if playing Settlers or Seafarers, replace some of the now-less-useful Soldier cards with interesting cards. Options stolen from CitiesAndKnightsOfCatan include Irrigation, Mining and Alchemist.
Variation to CitiesAndKnightsOfCatan: allow trading of the progress cards. Particularly often used on Mining and Irrigation: the typical quote goes "I'd get 2 ore from this, but you'd get 6. Will you trade me 4 ore for it?" They can still only be actually played on one's own turnr
Enforce the probabilities - to avoid the "I built on 6 and 8, but there have been more 10s and 3s this game!" feeling. It can still happen in the short term, but you know the numbers will even out in the medium term. Two options for implementing this:
Take 36 cards from two decks of playing cards, consisting of 1 each of 2 and Q=12, 2 each of 3 and J=11, 3 each of 4 and 10, ... 6 of 7, and shuffle them together. Turn over the top card when you need a die roll; reshuffle when you get through all 36.
Other house rules not generally used at GamesEvening
(although by arrangement I'm sure they could be on occasion):
Optional addition to the Counters on zero-production rolls rule above: allow trading of counters. If you allow it, it happens a surprisingly large amount - looks like Settlers rather benefits from having some cash at a resolution below the resource-card level. It's a significant variant, however.
Trading through ports permitted immediately after a 7 has been rolled and before discarding takes place
Playing with 2 players only: players start with 3 settlements (not 2), play to 12 or 15 victory points (not 10). Optionally also players are allowed up to 10 settlements, 8 cities, 28 roads (rather than 5 settlements, 4 cities, 14 roads).
Robin Hood: production lost to the hex the robber is occupying goes to the player with the least victory points.
SettlersOfCatan/CardGame house rule: roads cost only one brick and a wood, not the bizarre two brick and a wood which they appear to claim to require. Brick is already in short enough supply.
On the flip side, AlexChurchill is going off this one now, as it's rather easier to make a surprise road-and-settlement speed build, which can have a significant effect on resource income middle-to-late game, particularly when it's the final of the five settlements being grabbed by one player.