Pighixxx, from the Arduino forum, has created several pinout diagrams for the Arduino UNO and for several ATMega microcontrollers, such as the ATMega 328 and the ATMega 1284p.

These diagrams provide a clear picture about how to use each pin of the board and can be used as real “cheatsheets” for your own DIY projects. You can download them from here. Enjoy! :-)