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 and settings pages 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, some third-party developers have written some apps that you may be able to use to view the site; however, due to API limitations, not all features may be supported.
We used to offer official mobile apps, but those are no longer available for any platform. Existing installations of the official apps may continue to work, though they haven't been updated for a long time and are no longer supported for use. See How can I report bugs with or request features for the Stack Exchange mobile apps?
Still looking? Try browsing all the apps available at Stack Apps.