Dunno about you, but it sure pisses me off when I get a lot of Viagra and other dietary spam posts on my blogs, one of the reasons why I set up my own network of sites to get back links, saves having to resort to borderline gray hat / blackhat techniques just to get links, sure, more expensive option, but even more effective to boot when you have full control of each of your sites.
http://wordpress.org/extend/plugins/wp-recaptcha/
Once you've installed the plugin, you can then join up to this site below which will allow you to generate a KEY, for each new site there is no need to join up again, you just simply create a new UNIQUE KEY for each site you have a blog installed.
https://admin.recaptcha.net/accounts/login/
Enjoy!
