Basic 64

If you want to play with some c64 sounds then de la Mancha have you covered with the Basic 64.

Hobnox : Everybody Needs a 303

If you are bored and want to play with two 303's, a 909 and some guitar pedals then check out Hobnox Audiotools. It's all online which is a plus, but the downside is that there is no record button. But this is easily fixed by pressing record in your DAW or sound recorder etc. That being said though, in version 2.0 (coming soon) you will be able to save and load your work. Though to what extent remains to be seen.