Over the years, there have been a number of threads here regarding how to convert audio into MIDI, so I thought a few people might be interested in this.

Basic Pitch is a web based utility for converting single instrument sounds into MIDI. I think that it's been available for a good few months, but I've only just stumbled upon it.

I will say that it's fairly slow, but I'm quite impressed by the results.

A simple arpeggiated guitar figure was converted flawlessly. I then tried the introduction to Nic Jones' famed (at least amongst English folk guitarists) Canadee-I-O. Whilst not perfect, a bit of tidying up in a MIDI editor would turn out a pretty decent approximation.

Vocals are slightly more hit and miss. It's not that it does a bad job, but the entirely natural pitch warbling, grace notes and other ornaments, don't make for a particularly clean MIDI file. However, it's an excellent starting point, if like me, you're not that great at making accurate transcriptions by ear alone.

It's no good with chords, but that's not what it's aiming at. As an aside, the best software I've found for that is Cubase's recent Audio To MIDI Chords, but that, of course, costs money.