There are tons of modding options for the M3, just take a look at the sponsors forum for just a few of the things available. I've done a few mostly minor cosmetic mods myself so far... blacklines (they were previously on my 325i), matte black front/side grilles, painted reflectors (not installed yet), and tint (35% Formula One Pinnacle). Not much else planned aside from rims (probably not this year though) and maybe H&R springs at some point in the future.
Current: 2013 Mercedes C63 AMG Sedan Magnetite Black / designo Porcelain/Black Leather
RIP: 2011 BMW M3 Sedan Alpine White / Fox Red Novillo Leather
Past: 2008 BMW M3 Sedan Melbourne Red / Black Novillo Leather