well, it's my first post here, but i've been using arduino for over 3 years now and i love it!
anyhow, ontopic:
the title pretty much says it all:
rescanning the arduino user libraries on compiling/uploading, so you don't have to restart the IDE and re-open the project once you installed a library.
it's a feature i personally am missing on the arduino IDE and i think it won't be that hard to implement, as the IDE does it on startup already. 