AudioSauna transforms your web browser into a fast and flexible music production studio with built in synthesizers and live effects.
www.audiosauna.com

Free online Text To Speech (TTS) service with natural sounding voices.
Convert any English text into MP3 audio file and play it on your PC or iPod.
hwww.fromtexttospeech.com

Metronome Online
https://www.metronomeonline.com

Online ear training
http://ababasoft.com/music/teory_ear_training.html

Welcome to VirtualMusicalInstruments.com!
We offer you the possibility of learning and creating music by playing Musical Instruments online for Free using your PC keyboard and mouse control as the interface, or a touchsreen device. Our online musical instruments include the virtual guitar, virtual piano, virtual drums, virtual glockenspiel, virtual xylophone, virtual bongos and the virtual pan flute.
www.virtualmusicalinstruments.com

Online Audio Frequency Signal Generator
www.wavtones.com/functiongenerator.php

Touch Pianist - Tap in Rhythm
http://touchpianist.com

Online Tone Generator
https://onlinetonegenerator.com

 

Here are two of my favourite tools.

Chords and scales:
http://chords.jpglomot.com/#/home

Ear training:
https://tonedear.com/

PS. This one is a bit of a stretch, since it's an Android/iOS app for guitarists, but it's simply amazing:
https://play.google.com/store/apps/details?id=app.solotrainer.solo