I use a plugin that is simply called anti-spam. lol it needs no captcha and seems like it works cause I never get spam. I can't remember the reason I didn't use akismet but there are many easy options out there and believe me as you know it is necessary.
is akismet need to pay? does it help in blog registration or user signup also? because i get 400-500 invalid email from 'registered user' in blog.
Additional, to reduce spam, you can limit comments only for registered users, use a plugin that adds captcha on user registration form and comment form.
Use Akismet, it's the best spam plugin. Other than that, there isn't too much you can do. your'e gonna get spam on your blog even if you use plugins, trust me.