It's a free app available in Android Market that gives you access to some extra settings and battery information that were included in the original Android source repository but that didn't make it into the final firmware.
Spare Parts was made for Android 1.5 so some of the settings are outdated and/or don't make a difference on the 2.1-running EVO, but it's still useful because it provides application stats, basic battery info similar to what you get from Battery Indicator, and details about how the CPU, network, GPS, sensor, and other usage have affected the battery. The latter information can be filtered by the last time the EVO was unplugged, the last boot, and the overall total time, which is handy.
A few of the settings have a "1.5 ONLY" notice in their descriptions, so it's best to leave those alone. I don't think anything irreversible could happen, as whatever you change can just be changed back within Spare Parts, but it's probably better to err on the side of caution.