The stock Gmail app is one of the best mail experiences on a mobile device, but it can still be nearly impossible to tame particularly active inboxes. XonoMail Beta has just arrived in Google Play, and it promises to keep your email in order without a ton of configuration. Best of all, it's completely free and has no ads while in beta.
XonoMail is based on the open source K-9 Mail project. The official K-9 app doesn't get a lot of attention these days, and it's UI is extraordinarily outdated. XonoMail goes the Holo route (#YOLO) and pulls multiple mail accounts into one place.