Yesterday I publish a new blog theme and the spams arrived! Between 12 AM and 7 AM I received 12 spam emails in my Inbox.
Locating the free plan
When you open the Akismet signup page you'll see 3 paid plans. Just pay attention on the "Personal" plan. That you'll find "$0-$120/yr", so you must select that plan and in the next page you select to paid $0/yr for you personal use.
Fill the fields with your personal data and wait for the email with your API Key. After that just access you Askimet Settings page and set up your received API Key.
I hope help.