My friend and I are developing a web service startup at http://gnymb.us, and out of 70 unique visits to the site, we have had two signups so far. Is this good?
That is a low sample rate but sounds like a good conversion rate at such a low sample rate. In general compare your costs of aquiring conversions to revenue produced and try to compare it to the type of person your getting. So say your aquiring people from a specific ad word "development tools from anywhere" and it costs $5 per click then your unit revenue to break even needs to be 166 to $500 (example purposes only).
So conversion as a % on its own is not sufficient info to rate your business. I think as a first swag test without putting advertisement dollars down is a good indicator that your on your way though.
Generally the conversion is
Visitors (x visits) ---> Sign-up [Free] ( 0.1 * x ) ---> Paid customers ( 0.01 * x )
If you get 100 visits, you should get 1 paying customer.