To be frank, you can't really advertise a business without a budget! You should have a reasonable amount to invest in promotion, this is one of the fundamental aspects to any businesses success on or offline.

I would suggest the best form of advertising you can do (without any budget) would be posting in large communities and include a link in your sig. Perhaps do some design work in exchange for advertising?

Target your friends/local market first. Phone up local companies, businesses and offer your services, perhaps even do door to door sales? I can't think of anything else other than "word of mouth", nothing else is free!