We found out the hard way that maintenance, development and hosting does not pay for itself!
Being a not-for-profit website, it's important for us not to feature too many advertisements. Therefore, we only have one ad-space and we currently serve Google-ads in this space (example). You can rent this ad space on a per-day basis to post a few words on your product, job opening or the like. That way, your ad is the only one on the whole site.
The main audience is people working with, or interested in, interaction design, usability, interface design and the human sides of computer use and IT projects. Visitors are both from academia and from industry. Interaction-Design.org is one of the most well-known websites for people working in these areas.
You can verify these numbers with services like Alexa
Please make contact to inquire about the price. The price varies if, for example, your project is open-source or a public service.