It's hard to say what groups/organizations etc are the most popular on campus. I know a lot of people who do intramual or club sports, or participate in some kind of musical group. I'd say the most popular are any kind of organization that an audience can watch - so intramural sports or performing arts groups. The a capella shows and Sheer Idiocy shows (an improv comedy troupe) often get sold out, as well as the plays by the RPI Players.
I'm involved with the Pep Band at RPI. We support the women's and men's hockey teams by playing at home and away games. It's so much fun, because we're like a family - we spend basically every weekend during hockey season together, whether it's playing at the field house here, or traveling with the team to different rinks in the ECAC. Plus, we get the best seats at the field house - right behind goal!
As a freshman, everyone always left their doors open. People would stop by the door and introduce themselves in the first few weeks, and then once you got to know your floor everyone was friends with each other. Now that I'm a sophomore, a lot less people leave their door open because we've all made our core group of friends...plus I know that I'm hardly ever in my room because of classes and stuff.
Athletic events are pretty popular, especially the hockey games. It's really fun to go to them and be able to yell and cheer for your team. It's a great way to get all of your frustrations from the week out. Theater is pretty popular as well, whether it's the spring musical put on by the RPI Players, or the movie playing in the DCC on the weekend.
I met my closest friends through classes that we were having trouble with (I started going to extra help sessions with one of my friends my freshman year), through my roommate, and through joining Pep Band.
If I'm awake at 2am on a Tuesday, I'm either doing homework, or surfing the internet.
Every year, Big Red Freakout is one of the events I look forward to the most. It's the big hockey game, usually sometime in February. Everyone who goes to the game gets a "Freakout gift"...usually something like a noisemaker, team pennant, or baseball cap. Everyone gets really into the game, paints their faces, hold festivities all day, etc. In the Pep Band, we have a lasagna dinner, then all of the alumni and current students play at the game in one huge, awesome sounding band. Afterwards there's a student-alumni mixer at an alumni's house in the area, with awesome food, and then an after party at someone's apartment.
Fraternities/sororities aren't the main focus on campus, which is nice. You know they're there, but the majority of people here aren't in one. During rush week there are a lot of fun events open to everyone, like cookouts or volleyball games. I know a bunch of people in sororities and fraternities and it seems that here, it's just another way to connect with people and really have a strong group of friends.
Last weekend, we didn't have any Pep Band events so I cooked a steak with my boyfriend, played pool in the Union, and went to a Wing Tasting charity benefit in the Union that was DELICIOUS.