Boston Celtics
Baltimore Ravens
Atlanta Braves
Ohio State Buckeyes - football
Cincinnati Bearcats - football
I was pretty big into the UFC for awhile and still follow that, though not as much. Since it isn't a team, I didn't list it.
I also watch the Team USA National Soccer Teams, but don't watch much soccer outside of international competitions. The Columbus Crew is the MLS team I follow more closely and I don't watch any of the European teams.
I don't really watch hockey much anymore (watched it way more as a kid), but the Columbus Blue Jackets would be my team these days (as I kid there were not Blue Jackets and I was a Bruins fan).
I don't watch NCAA basketball much until the tournament these days, but in college was a huge fan and went to basically all of the UC games (the Kenyon Martin team was a lot of fun till he got hurt) and also follow the Buckeyes (#4 in the country this year so far).
Being in the Cleveland area, I get inundated with the local teams so I at least pay attention to the Indians (or whatever they will be), the Cavs, and the Browns (though that one is harder since the Ravens are my main team).