LinuxSampler provides a sampler audio plugin suitable for use in Ardour and Bitwig Studio. It can load GigaStudio/GigaSampler, SoundFont, and SFZ sample instruments. Currently, there is no other plugin that loads any of these three formats.
Open Source: Yes
Users: The same userbase that finds Ardour and Bitwig useful may also find LinuxSampler of use. Honestly, not that many people (not the 'average user's), but a group I feel should be catered towards.
Depends on libgig
Related pieces of software are:
gigedit, an editor for one of the formats supported by LinuxSampler
QSampler, a QT based, partially featured front-end to LinuxSampler, or JSampler, a Java based, fully featured front-end to LinuxSampler.