Or, to quote the not-so-frequently-asked-questions-although-frequently-ignored-answers:

"You may not advertise a product, service, or web site in any post
You may advertise anything you like (as long as it doesn't break any other forum rule) in your 500x125px signature space. Users who appear to have registered for the excusive purpose of advertisement will be banned without warning."

"Stephen Wolfram is the creator of Mathematica and is widely regarded as the most important innovator in scientific and technical computing today." - Stephen Wolfram