Here are a few tools that have been a great use to me when working on my own BMWs.
Torx / Inverted Torx Set
When working on any BMW, you will need a good set of Torx sockets. This set includes both Torx and Inverted Torx along with Security Torx. Security Torx have the small hole in the middle of the bit. My e90 335d had these type of Torx bolts on one of the under trays.
Injector Puller
This mini slider hammer works great for pulling stuck Injectors. It includes a couple of adapters for different size injectors. I have used it on both my N54 and M57.
Stretch Belt Install/Removal Tool
This tool makes easy work of removing and installing the Stretch Belt for the M57 AC Compressor. It is magnetic and attaches to the crank pulley. As you turn the crank pulling, it will push the belt off of the pulley. To install the belt, set the tool on the crank opposite of the removal and turn the crank to pull the belt on to the pulley.