I read a lot about small hacks that can help when gaining traction with very small budget. Some of the most commons strategies are:
Profile signature in related forums. No cost, but some forums have limitations when promoting services or communities.
Pins or badges. Very low cost. If you use public transport or places with with lots of people, just attach the badge (with a logo or url) to your suitcase. Stickers in the car or laptop can work too.
Job announcement in large job boards without real intention to recruit. Well, this one is unethical, but widely used by companies.
Your personal email signature. Create a signature that redirect to your service.
Be part of a community like Quora or sub Reddit and offer your expertise, and periodically add some mentions about you service.
Guest posts in amateur or small blogs. In my experience you need to start always with low traffic blogs. Otherwise they will ignore your request, unless you are well known to the audience of the blog.
Comment news about your audience in big websites using a profile that redirect to the service. Example, using a Disqus profile for that. This is very effective strategy.
Well, I think it's interesting create some guide about zero cost strategies to people that try to create a new community, with a very limited budget.
My idea is just turn this post in a community wiki. If you think that is inappropriate just delete. Otherwise, please share some of the strategies you use with a very low budget, near to zero.