Attempt #1:
http://www.stereotool.com/download/vst_stereo_tool.dll
Please let me know if it works. I have briefly tested it with a few free VST tools, and with a Winamp VST plugin bridge (lets you use VST plugins in Winamp). The VST bridge version sometimes crashes when unloading the plugin while music is playing... So be careful with doing that. (I'll look into that later).
Also, parameters are not supported for VST host applications. You can only edit them as in Winamp and save them as in Winamp, the VST host application cannot (yet!) access them.
Edit 2008-08-18: Some parameters are now available. Please don't rely on them yet, their IDs might still change.
[Note: This is a VST plugin dll file without installer. Please save it to your VST plugin directory, or select it manually from where you saved it if your VST host application allows doing so.]