Custom >NAME group for components in EAGLE
When you are editing your device in your library, there is a prefix on the bottom right of the page.  This is where use choose it the device will be E1, E2, etc or R1, R2, etc.. or something else of your choosing.

For example, here's a screen shot of a simple button switch in my library that I have given a prefix of "S" so when I add them to my projects they are S1, S2, etc..