Defensio is a comment spam prevention service, similar to Akismet. It's from a company based in Montreal.
An awesome and awesomely simple service that allows one to create a disposable email address on the fly, anything @mailinator.com and then check it later. All email is semi-public. Very cool.
WP-HashCash implements the hashcash approach to spam prevention as a WordPress plugin.
Controlled Mail is an outbound mail service (SMTP provider) with a very strong focus on genuine mail delivery. They accept no bulk mailings whatsoever and support SPF and DKIM by default. They also prevent cross client forgery.
FuseMail is a hosted mail service which includes webmail, IMAP, POP3, calendar functions, shared files, groupware, and more. All for $2/month/mailbox. Great.
Hashcash is an anti-spam technology which requires the sender to compute a partial hash collision (which takes a predictable amount of time) in order to "pay" for their email. On the flipside, it's very easy to verify the hash.