livefinder lists the YouTube live streams that are on right now, ranked by concurrent viewers, so you can see at a glance what’s most watched. It also lists upcoming streams by start time, recently ended ones by views or length, and the channels behind them.
You can narrow streams to a language, and search live streams by title, channel or tag. Viewer counts are YouTube’s own and are updated every minute while a stream is live. It’s free, with nothing to sign up for.