I don't know how they do it, but Blue Cat puts a lot of care into their freeware plugins. Gain Suite is one of the best ways get automated mixing and volume adjustment operations done with a single slider. The suite sports a new user interace and now sports unod as well. The whole skinny from Blue Cat:

Feature Set:

• Mono, stereo and mid/side gain.
• Link several instances of the same plugin and control several plugins from a single user interface with advanced link capabilities.
• Smooth Bypass: activate/deactivate the plugin with absolutely no noise.
• Undo/Redo.
• Import/Export presets and share them between the directX and VST versions.
• Integrated window opacity support (make the user interface of the plugin transparent).

Blue Cat Audio Standards:

• Native DSP code for optimal performance.
• Skinnable user interface.
• DirectX and VST plug-in formats support.
• Smooth Update: silent (zipper-free) parameters update.
• Full automation support (all parameters can be automated, with smooth and continuous update)
• Real time MIDI control with advanced settings (MIDI channel and CC numbers, with complete response curve control) and MIDI learn.
• Any sample rate supported.