Nethack is great, if not a little vanilla, and it's extremely popular even though it was made decades ago, but it's a little hard to get into and has a lot of nuances that can be problematic to sort out.
Dungeon Crawl Stone Soup is a pretty decent "new" roguelike that is pretty user friendly and has a pretty big playerbase. It's pretty streamlined and it wouldn't take much to jump in and start playing.
There are literally hundreds of other free roguelikes out there, but these two are my recommendations.
