Need a battery for your Arduino that is high enough to regulate the voltage to down to a solid 5V? Well here comes the 9V Snap to 2.1mm Barrel Plug Cable to your rescue! It allows you to plug in 9V and 7.2V rechargeable batteries to anything with a 2.1mm Barrel jack on it.
The DTE RS-232 module is a tight and tidy way to convert RS-232 (computer-level) signals down to a microcontroller level signal strengths. DCE means it's configured as a "Data Communications Equipment" (with 9 holes).