According to a Proz poll.
Some of the top choices: Proz membership, networking, software/hardware, education.
I personally liked the option "hiring a proofreader", I'm sure it's an investment that pays.
http://www.proz.com/forum/poll_discussion/263461-poll%3A_what_is_the_best_investment_you_have_ever_made_for_your_business.html
