Android: If you have multiple email accounts from multiple providers, you probably also have multiple apps on your phone to check them all. SolMail is a new email client that wants to bring them all under one roof, without sacrificing features for the convenience.
SolMail is new, so it's definitely a bit rough around the edges, but the developers are actively working on it and taking feedback. The app currently supports any IMAP or POP account, with special support for Gmail, Hotmail (Outlook), AOL and Yahoo! The devs are working on Exchange support next. You can swipe to archive, star and filter, filter by attachments or by read/unread, and by messages sent directly to you as opposed to those sent to a group. You can add a mobile signature to your outgoing messages and tweak the reading font size of your emails to fit your screen.
The app isn't perfect though -- SolMail passes credentials directly as opposed to using OAuth, which means it will be out of the question for more security-minded folks, or people who use their Google or Outlook accounts for business. It also doesn't support Gmail's push notifications, although you do get notified of new mail in any account through the built-in Android mail widget. Finally, the app is optimised for Android phones and not tablets (yet), something the devs say they're working on.
Those things aside, SolMail is a sharp-looking app that makes reading and organising mail from multiple accounts easy. Colour-coding your email and filtering it goes a long way towards keeping everything organised, and the gestures to archive and move forward and back among messages is nice too. The "stickers" feature is pretty much emoji for your email, so if you're into that kind of thing, enjoy. SolMail is free and you can grab it at the link below.