If you didn't know, every Stack Exchange site has a responsive layout - meaning that if you access the desktop site on a mobile device, it will render in a mobile-optimized format. Most views, including the homepage, questions, and search render in this view, though some pages such as user profiles, settings pages, reviews, and revision histories don't.
Additionally, each site has a mobile site which provides an altered way of viewing each site, specially designed for mobile devices. This provides a low-bandwidth view for slower or metered connections. You may automatically reach the mobile site just by visiting it from a mobile device, but you can also get there manually by clicking the "mobile" link in the footer.
The official mobile sites provide almost all functionality as full sites, including creating new questions and answers, voting and commenting. Some features are not implemented on mobile sites yet, like reviewing, profile editing, and Developer Story (SO only). Certain pages don't have a mobile theme, either, like flagging history.
If you prefer a native app over the mobile site, there are plenty of apps available based on your device.
Note: The official mobile apps are no longer under active development, so you may encounter some bugs or glitches. New feature requests or bug reports for minor (non-critical) bugs may not be handled for a long time.
Available on the Apple App Store:
Available on Google Play:
Available in the Windows Store:
- There is not currently an official Windows Phone app, and there will not be because of the platform's discontinuation.
Still looking? Try browsing all the apps available at Stack Apps.