This is a 9V battery pack with on/off switch and a 2.1mm barrel plug, you get both parts and will have to solder them together. Use this to battery-power your Arduino (or other electronic projects).
This is a really nice power supply. It's a switching DC supply so its small and light and efficient. It is thin so it fits in power strips without blocking other outlets.
It's a nice quad-AA battery holder, with 6" leads.
A nice 6AA Battery holder in black plastic with wire leads.
The ST Micro L7805CV is a 1.5A, 5V positive linear regulator. It is the stronger older brother of the 78M05 but ironically it is the same size and shape, go figure. It may look like your everyday 7805 but looks can be deceiving!
Sometimes, voltages collide. When you have parts that expect/speak at different voltage levels, you have to build a mediator to smooth things out. This SparkFun BOB-10403 uses PCA9306 two-channel bidirectional voltage-level translator on a nice, small breakout board.