When you install this plugin, without setting up any config files, all Endermen will drop the block they are carrying when they die. A setting can be changed in the config to disable Endermen from picking up blocks completely (new in version b1.2.1).
By default, if an Enderman picks up grass, it will be swapped out for dirt, just like when a player mines grass, to prevent people getting grass without silk touch. The default config file can be easily edited to disable this block swapping completely, or to add your own rules to switch certain Material IDs out for other Material IDs (for example you could make an Enderman who picks up a sand block, end up with a diamond block instead.)
That's all there is to it. I created this small plugin to replace a similar one which I was using which the author didn't update, and have added a few of my own features.
If you are using or have tried out this plugin, I'd love to hear your thoughts on it. Post a comment and let me know what you think, and if there are any new features you would like to see.