Funnily enough, this applied to the company I work for this week. We're running a small email marketing campaign for one of our clients to ~500 email addresses. They're known addresses and know of our client so are warm leads, even so.. from that two of them went onto our client's site, found our logo in the footer and we got jobs from them. One, an email marketing campaign, and one a website! Both of them had also contacted our existing client about us and got a recommendation. It costs nothing, and providing your client is happy for your logo to be on their site, if you get a single job out of it, it's worthwhile.