The service has been free for nearly four years, and we don't see a reason why it won't be in the future. We pay market rates for bandwidth and servers, but the cost is covered by donations and the google ads. The service has *always* been "in the black", except for the first month, where I did actually have to cough up the $10 hosting fee (that's how much it cost per month until July 2003, when we outgrew the web hosting account, and had to move to "real" servers). We also pay for most of the domain name registrations, but that isn't too much.
Beyond that, we've designed the service in a way that's calculated to minimize costs. Browse the forums, and you'll see us pushing back on requested features that we believe would be too expensive. The auto-expiring nature of spamgourmet's email addresses allows us to handle a lot more users that we would otherwise -- the way the statistics work out, over 9 out of 10 email messages is "eaten" by spamgourmet, and it's much cheaper to ignore a message than it is to forward it. Likewise, we'll reject a new feature if it looks like it'll give rise to a big support burden.
The site and service have a "geeky" presentation -- that may change soon, but it has been important so far. We believe it has had the effect of scaring off people who would require a lot of support. The core of users that have signed up so far are, well, geeks -- most don't need support, and are also willing to give support (on this board and elsewhere) to users who do. If we had done a flashy, well promoted offering before building this base, we might well have been overwhelmed.
The people part of the service (development, support, language translation, etc.) is performed by volunteers, but, to my knowledge, none of us have that 'overwhelmed' feeling. I personally think it's a lot of fun, and I hope the others do, too. When we need to, we take a break - development slows down, etc. (obviously, the service stays up, but that's provided by computers, not people -- the computers don't get breaks
). The support burden has been small enough that it has never posed a problem -- we've never felt the need to take a break from support. This board, as I mentioned, and other places like newsgroups, help, too. It's incredible to surf up and see that a question has been asked and answered by people I don't know. More than once, the unknown benefactor has perused our open source code to come up with the answer.
Other aspects of the service have been donated by companies, like our test server, and soon, backup DNS. Additionally, some people have donated domain names. As I mentioned above, though, the core of services necessary to make spamgourmet work are fully paid for -- this insulates us from policy changes in donor companies, and the like.
That's it -- there's no investor who will come demanding a return on investment. There's no capital cost in terms of offices, leases, etc., and no debt. We've spent (and intend to spend) exactly $0 on marketing, and feel no pressure to grow the service artificially.
I hope that answers your question --- I'd be happy to expound further and to entertain any more on the subject as well.