LinuxSampler is a professional grade software audio sampler that aims to deliver performance and features at par with hardware sampler devices. LinuxSampler itself does not have a graphical user interface, however, there are frontends that exist for it, eg. Qsampler.
There is also an LV2 version available. To find out how to use this, check out Linuxsampler and Ardour: Using them together.
Carla plugin host also has support for loading sample libraries, using LinuxSampler as it's backend.
Note - There is controversy over the licence of LinuxSampler and its compliance with the GPL, as LinuxSamplers backend restricts commercial reuse of it's code. While most of LinuxSamplers components are free, its backend is not.
The Linuxsampler developers are currently working on their own custom licence.
Salamander Grand Piano sample library loaded in Carla, which uses LinuxSampler as it's backend for sample library support