QuicKeys is a piano GUI to control other VSTi's within your sequencer.

You load it up as a regular VSTi or as an effect, then click on any track in the arrange window that is routed to another synth/VSTi and now you can play this synth by clicking on QuicKeys' virtual keyboard (or with your own computer keyboard). MIDI processing/converting options are included as well as sending CCs, a MIDI LFO and an auto-chord feature.

Note: functionality of this plugin is largely dependent upon your host !