[sverige]

The best is G4FON's Morse Trainer, requires Windows, works on Wine.

http://www.g4fon.net/

Edit: You'll have to navigate to the Morse Trainer link from the main page.

Also, it's the best because it has knobs for all the variables -- speed, number of characters sent, even options to add QRM (interference) and variable sending speed (imitating very well someone with an unsteady fist).

As an aside, it's pointless to learn code visually. It's only really useable as an auditory messaging system.

[SiVal]

only really useable as an auditory messaging system

What? When I was a kid, it was common for camping flashlights to have pressure buttons on the side, which was intended for (and we used for) morse code. Ships used bright, shuttered lights for Morse code, soldiers used it, scouts used it, kids communicated with friends in the neighborhood at night.

[sverige]

OK, yes, but it's still the rhythm of the flashes that are being interpreted, not _ .... .. ...

Plus, I am willing to bet that far more messages using Morse are sent over radio than with lights.