I personally joined Socialist Alternative. I think we do good work, lots of focus on movement-based politics, workplace organizing, that sort of thing. It was before I joined, but we won the first $15/hour minimum wage in the US, which I think is a pretty big achievement. Plus we have good, principled stances on the Russian and Chinese ruling classes, (just because the US ruling class is bad does not mean the ruling classes that oppose them are good, or bastions of left-politics) which is unfortunately a pretty divisive issue on the left.