Spamihilator - The free Anti-Spam-Filter Spamihilator removes more than 98% of spam mails before they even get into your inbox.
Hiver - Manage Customer Support, Sales and Projects right from your Gmail inbox. Collaborate with Shared Labels, Shared Contacts, email notes, email reminders and email templates
Mailwasher - Over 8 million people use MailWasher as their spam filter
Clean Email - Clean Email is an online service that empowers you to take control of your mailbox.
SpamAssassin - Use this program to "Unspam" your mailbox(es).
Nylas Mail - The Nylas Cloud API powers your application with email, calendar & contacts features. Built-in features for better email, calendar, and contact management.