There are three expansions for Settlers of Catan which are used for 5-6 players to play instead of the normal 3-4: one for the original game, one for Seafarers, and one for Cities and Knights. This is pretty annoying because it means you need to buy a separate boxed set for each expansion if you want more players, instead of just one 5-6 player set which works for all three versions of the game. However, the 5-6 player expansions add a lot more fun to the game and are worth the extra money.
The 5-6 player expansion to the original Settlers of Catan is the first expansion I recommend players buy, even before Cities and Knights.
The ability for more friends to play at the same time is awesome and makes games last a bit longer. The expansions add more land, water, and port tiles to make the board larger, along with a new sets of numbers to use when making the larger maps. They also adds new colored pieces (normally green and brown) for the two extra players. When playing with a 5-6 player expansion, the rules of the game stay the same, with two main exceptions. First, the steps to build the board are different, since they needed to be adjusted for the new size of the board. Second, and more interestingly, after every player finishes his turn every other player gets a chance to build anything they want. It doesn't take long to adjust to this new style of building, and you still look forward to your turn since you are able to trade with other players and the bank at that time.
The expansions normally cost $20 and can all be purchased at Amazon.com. If you enjoy Settlers of Catan then I can't tell you enough how much fun the 5-6 Player Expansion
adds. Nowadays, I rarely play any games of Settlers without using it. The Seafarers of Catan
and Cities & Knights of Catan
5-6 player expansions are also great to have if you enjoy either of them. The Seafarer's 5-6 player comes with a few more scenarios to use, along with the expected tiles and pieces.