With the latest AmigaOS 3.2.2.1 you can open ADF files. But if you like to use other versions of AmigaOS 3.x, then Isomount is a fantastic tool to have.
With Isomount you can mount several types of files as devices. This is very handy for ADF files as they are used in the UAE emulator.
ISO files are also neat to mount if you don’t have a physical CD-ROM drive connected to your Amiga home computer.