Soundtrap allows you to make and edit sound and music online.
Audiotool is more like Reason than a traditional desktop sequencer, so if you prefer that style this is a great option. Audiotool contains a host of very high quality samplers, synths and effects that you can combine together to create some great sounding tracks.
Audiosauna is a fairly traditional sequencer, but it’s more for note triggered soft synths rather than audio loops. There’s a number of soft synths available as well as a mixing rack and effects.