The Arduino Pro Mini is SparkFun's minimal design approach to Arduino.  It's essentially a 5v Arduino that runs an 16 MHz bootloader on a super small, super thin board.  There are no connectors attached - allowing you to add whatever connectors or wire you want without too much trouble.

The goal of the Pro Mini isn't to replace the Arduino so much as bring the cost down for more advanced users.  The Arduino Uno is still the inimitable board but if you're comfortable working within voltage limitations, don't mind off-board USB, and want to customize your connectors, the Arduino Pro Mini's a great budget purchase. You will need an FTDI friend or FTDI cable to program your Pro Mini, and we also recommend some plain header to solder it for breadboard use

Check out SparkFun's tutorial for more info on how to get started with the Arduino Pro Mini!

If you would like an Arduino IDE compatible board with USB, also check out the Pro Trinket 5V!