This comes up a lot, and I just want to give my opinion and tell you a little about my experience with this subject. When I was 18 I started an e-commerce website. Back then, everyone was starting up online companies and it was right after the dot-com bust so it was hard to even get people to take you seriously if you told them what you did.
Well, we started up as a general partnership but myself and the other owner called ourselves "President" and "Vice President" on our business cards.
It's an easy mistake to make when you're just getting started in business... you want to sound professional - maybe sound a little bigger than you are. But the reality is that legally unless you're incorporated (or an LLC) there isn't a position of President, CEO, etc.
It actually makes you look even more like an amateur for trying to call yourself one, because anyone with half a brain will see right through it. See, if you were incorporated your company name would have to have an indication (such as Inc., Incorporated, Corporation, LLC, etc.).
I'd recommend just calling yourself either Owner or give yourself a position like "Creative Director" or "Head of Web Development." They're very professional sounding titles, and won't draw any attention to you like "President" or "CEO" would.
The funny, whimsical titles never sat well with me. "Web Guru" and the like just don't sound professional. You might get a few cutesy comments aboutt them but in the end I think the potential for a prospect to think that it was immature or unprofessional outweighs the benefits (are there really any?).