@melissasage for algorithmic synthesis the two biggest are #puredata and #supercollider.

#pd is boxes and wires graphical user interface, similar in some ways to #maxmsp . It also has external plugins for graphics and video.

#sc3 is text code, and has more support for things like multichannel signals with easy to use libraries for #ambisonics for spatial sound.

Both have active mailing lists.

Other software like #tidalcycles for #livecoding patterns use SC3 as a sound engine, and are probably easier to get musical results with quickly.

Pd has #netpd for collaborative jamming online, I don't know how busy it is but it has some nice sequencers instruments and effects, all built in Pd.