Let's see: we already have an idea here -- "banner exchange" on our websites. Very good!
Here is another idea: blogging, blogging, blogging and networking with other bloggers who can promote your idea on their blogs (start at http://blogsearch.google.com).
This works miracles because is based on a deep human insight: sharing news with other people to look knowledgeable. Take my word for that -- I happen to work in digital advertising and I have a good feel for what works and what does not work, based on experience and instinct.
Another reco is to visit http://www.touchgraph.com/TGGoogleBrowser.html
and take a look at your website's graph. This will give you some ideas with regards to whom might be in your area of interest and could potentially help you. At minimum, the Touchgraph is a very cool tool!
I would do a total injustice to Word Of Mouth if I didn’t mention this powerful viral marketing tool. Where and when appropriate (please, keep this in mind), try to build a referrals system and offer incentives for other people to recommend your business to potential clients and give you trusted referrals.
Modesty aside, mybeezoo.com is a must-see (soon to come) on-line live networking tool/community who’s sole purpose is to identify new customers for its members. Stay tune for this one!
Let's hear some other ideas and have a wonderful week-end!