The best night spot in PGH. Hot Mass is everything you could want from an after hours. $15 to get in, always solid DJs on the bill, sound system (recently updated) is world class, and they're BYOB friendly.
The dance floor is just the right size and there's a designated smoking room for the cigarette crowd. Usually gets to capacity around 2:30am so if you don't want to wait in line then arrive a little earlier. The club itself isn't big by any means, so it can be tough to move around, but you'll be fine as long as you're not going to the bar every 10 minutes from the dance floor.
I always tell my friends they need to come out to Mass at least once. It truly is a special place with a good vibe.