Here’s a list of OpenVPN clients I recommend for various platforms.

Windows

Use the official open source community client.

Mac

Use the open source Tunnelblick client which is both OpenVPN and GUI in one app.

Linux

Use your distribution package or compile OpenVPN from source.

Android

I recommend this unofficial but open source OpenVPN app for Android. It has more options and can do more than the official app.

iOS

Use the official OpenVPN app.