You.dj is fun live mixing platform that allows you to search for beats and instruments in Youtube and Soundcloud as well as the already listed samples.
Looplabs is a more complex looping program where you have to follow the key (C, Cm, E, Dm etc) in order for the music to fit with each other. There are lots of sounds to play and mix with.
Sampulator is a simple beatmaker online that i limited but sounds great.
Create your own project and get to producing.
Splice is an easy sequencer that allows you to build 'sweet' beats. There is a paid version that is not needed.