Also on Jan's Midipage is a Non-MIDI page.
This page is of course about music and keyboards. Midi is not central, but indirectly it has a little bit to do with it.

Below are a few web links that allow you to play realtime with a synthesizer.
Often also available as a free VST tool that you can install on your computer, together with a VST Host.
(See also the VST page)

Click on one of the images to go to that website.

DEXED virtual synthesizer. Can be operated live via the web browser

The mega popular clone of the DX7

Once you are on the website, you will probably only see a keyboard.

1. Click on "Toggle Frontpanel" to see the entire synthesizer.
2. Choose a sound bank (is already on a sound bank by default)
3. Choose a "Patch" as a separate sound setting. (sound)

TrueGrid virtual modular synthesizer. Can be operated live via the web browser

TrueGrid is a modular synthesizer where you can make connections using "patch cables" to create a sound.
A complete synthesizer can be made by means of modules (each with its own property).

Click "Enable Audio" to turn on the sound
If you do not want to experiment with the cable connections yourself, you can opt for one "Patch#1" 

PatternSketch virtual drum sequencer. Can be operated live via the web browser

PatternSketch is a drum sequencer.
All rows represent different drum instruments and all columns represent one of the 16 positions of a running rhythm.
Fun to try. Sounds pretty good.

Have a lot of fun.