Not a VST - but a really useful little free virtual midi keyboard - that is really handy when setting up/testing racks or when writing scripts.
I have been using it for about 7 years and it has never given me any aggravation. I use it all the time when I am writing scripts and the like when away from real keyboards (mainly because I have an affinity for comfortable chairs when doing this sort of stuff!).
There is also the “MIDI Tools” application from Mountain Utilities which is for OSX and Windows. It does not just include a virtual keyboard, but can send all kinds of CC messages, SysEx messages and various other things. https://mountainutilities.eu/miditools