Airwindows has released SoftGate, an AU Universal Binary plug-in for Mac OS X to help with sampling and sample instrument creation. It costs $49.99.
The purpose of this one is more like mastering noise reduction or de-essing. As the gate closes, it also rolls off highs - instead of going for outright gated effects, it's designed to not really close completely, just to soften and suppress hiss and background noise. This is for the purpose of working with instrument samples that must decay to silence, or reverb impulses that do likewise.
Bright Threshold and Darken Speed should be pretty obvious, and Base Silence is the control that's like a de-esser: it's the depth to which the plug-in can gate. In noise reduction often it's better to just step on hiss and noise rather than trying to wipe it to digital black, and this plug-in is all about letting you transition from a fully bright, unaltered sample to an attenuated and highs-softened one seamlessly.
Sample instruments you make will have a warmer, less fatiguing sound due to the softer background hiss, and reverb impulses will decay into the distance better - you can dial in your preferred decay, and there's a SoftGateLinked plug-in included so you can work with stereo files, as well as mono files, properly.