You can use akismet for free, as long as your site doesn’t produce more than 10.000 pageviews a month and you use no ad- or affiliate-program on your page.

@Christoph - I've been using Akismet since I started using WordPress. When I started, I maybe averaged 50 pageviews a month. Obviously I was far under the radar. Now I'm averaging about 2,000, but still not in the clear by any means. I also recently added Google Adwords to my site ... and I've made an average of $0.005 per month in ad revenue.

Does having Google Adwords mean I must start paying $5/month for the service? It's confusing to me, since it is a personal site with relatively low traffic. If I were actually making money, real money, from my advertising I'd have no problem paying for the service. But when you start adding the costs of premium themes to web hosting to domain name registration to this new-ish charge for Akismet, it becomes a bit ridiculous.

I also have to wonder about WordPress multi-site. I am in the process of splitting my one blog into several topical blogs on a single network. Does having multiple sites where I used to have one magically bump me into the "professional" category? It's the same content, just divided over subdomains rather than category pages ... but if you click the "I need Akismet for multiple sites" option, it suddenly becomes a paid service.