I'm a native Eugenian, so I'll take a crack at this. I left Eugene a while ago; I tried to keep current, but who knows what may have changed:
music festivals: This depends on what kind of music your interested in. Eugene is pretty well known for the Oregon Bach Festival each summer, which I believe is one of the premier Bach events worldwide. For pop/rock/folk, in the direct Eugene area the best thing going on is the Oregon Country Fair, where aging and not-so-aging hippies converge for a drug-addled weekend of folk crafts and jam bands (this was never really my scene, though it's an interesting event for anyone). A day's drive to the north at the Gorge Amphitheatre, there's the Sasquatch Festival each summer. This usually has an excellent lineup if you're more interested in indie pop and rock. There's also the Eugene Celebration, but I always found this pretty boring. Local groups also play frequently at the Saturday Market.
venues with frequent concerts: Again, it depends. The Hult Center for the Performing Arts is where all the classical and a lot of the jazz happens. For rock'n'roll, try John Henry's, the WOW Hall, Sam Bond's, Indigo District, with larger stuff happening at the McDonald Theatre. The Shedd and the Cuthbert Amphitheatre also have stuff going on.
what type of music seems to be coming out of the bars: This one I can't answer. Been away too long.
art festivals: This is hard to say...When I left Eugene I was not necessarily a big connoisseur of the visual arts, though Art and the Vineyard was the one people talked about the most. There are also rotating exhibits at the Hult Center, an art museum at the University, and numerous independent galleries (though remember Eugene is not a major city).
indie theaters: Movies or live theatre? For movies, it's unfortunately pretty much limited to the Bijou, which doesn't usually get first-run independent movies. This situation may have changed in recent years, though I'm not sure. Check EugeneWeekly.com to see if there's any special events hosted by coffee shops, bars, etc.