This is a card holder that fits Catan cards. Glue three of them side by side and you've got yourself six slots in which to vertically place resource and development cards. I kept having troubles printing out the really nice one by MaxGjr:

http://www.thingiverse.com/thing:399805

This smaller one prints way more easily for me.

December 6th, 2015 update: added another version that allows you to vertically display the Largest Army and Longest Road cards behind all the others. It also features the option to connect them by gluing or bolting together (thread a 190mm x 4mm rod at both ends)

January 1st, 2016 update: if bolts and glue ain't your thang for bonding these parts together, tejawood made a nice snap-together version here:

http://www.thingiverse.com/thing:1238868

January 27th, 2016 update: Added a 3rd version - it features extruded bumps which allows cards to sit more upright and not lay against cards behind them.