It depends a bit on the sport.
The last I checked, NFL Sunday Ticket is available streaming with a Playstation (no satellite subscription required). And if you can wait, I believe they have a "rewind" option online where you can stream full games a day or two later for only like $40 a year.
The NBA offers an online streaming option. I subscribe to Ballstreams, a European rebroadcast that is cheaper and more dependable than the US version (it's not sanctioned by the NBA, but still legal and has been around a while). I'm not a hockey or baseball fan so am unsure about all of the options but I know they exist.
College football is tricky, as ESPN seems to have a monopoly on bowl games. You can either sign up for the new Sling TV, borrow a login from a friend, or go to the various "gray-area" streaming sports sites on the web.
I personally use the questionable sites and have had great success. They have many more sports than you'd normally see (Premier league is cool, DW was even watching rugby and cricket recently). I recommend Adblock as well as Chrome to control auto-play on flash plugins. Most importantly, never download software to stream anything. But plug it into a TV, and the quality is surprisingly good.