Yes. Our NearlyFreeSpeech.NET DNS service allows unlimited subdomains under a single domain name at no additional charge.
Please be aware, however, that we do not use the subdomain-is-subdirectory hack (unfortunately) made popular by a certain brand of web hosting control panel software. You can use our service to create multiple independent sites and then assign or remove names from one or more NearlyFreeSpeech.NET DNS domains at your discretion. There is no connection between a site and a domain name or subdomain other than what you create, and there is no correlation at all between subdirectories of a site and subdomains of a domain.
Subdomains can be associated with sites we host by adding them as aliases. If you're a member, see this member FAQ entry for more information.