Most cable TV packages include access to watch sports online. It is also possible to watch sports online without a cable subscription, but options are more limited and may require some sacrifices. I will go over all of the major sources of popular sports here in the US that are available without a cable TV subscription.
Watch sports online with a subscription
Sling TV is subscription streaming service that is a great way to watch sports online without cable. It starts at $20 per month and provides access to over 20 channels including ESPN, ESPN2, TNT, and TBS. ESPNews, ESPNU, SEC Network, and more are available for an additional $5 per month. ESPN is the cornerstone of the service, with TNT (NBA, March Madness) and TBS (MLB) providing additional live sports. Sling is available on smartphones, tablets, computers, Xbox, Roku, Fire TV, and more. Try Sling TV free for 7 days.
Playstation Vue works just like Sling, but offers different channels. Sports channels include CBS, FOX/FOX Sports, NBC/NBC Sports, Golf Channel, and regional sports networks depending on location such as Comcast Sports, YES, and Big Ten Network. Packages start at $50 and go up to $70 and include more channels than Sling TV. It is only available in a few major cities on PS3 and PS4 for now, but should be expanded soon.
The big four American professional sports all offer subscriptions to watch sports online. These products are marketed as offering access to every out of market game, which means they black out any games available on TV in your area - whether it is on a cable sports channel, broadcast channel, regional sports network, etc. This also means they don't include any playoff games. Get more info on NFL Sunday Ticket, NBA League Pass, MLB TV, and NHL GameCenter.
Watch sports online free
The CBS Sports website and mobile apps have free live streams of most weekend sports broadcasts from CBS, with the exception of NFL football. This includes SEC football, college basketball, and PGA Tour golf. Watch on iPhone, iPad, Android, mobile website, desktop website, or Apple TV.
watchESPN offers ESPN3 free with most broadband internet subscriptions. ESPN3 includes ABC broadcasts of NBA games, college football and basketball games, alternate camera angles from big events, Spanish broadcasts of ESPN broadcasts, international sports, and much more. It's a great way to watch sports online for free and is available on a ton of devices including iPhone, iPad, Android, computers, Apple TV, Roku, Chromecast, Fire TV, Xbox, and more.
NFL on Verizon mobile
Verizon is the exclusive provider of live NFL games on smartphones, and it is available free through the NFL Mobile app with a Verizon MORE Everything plan. This includes access to all primetime games (Sunday, Monday, Thursday) and local Sunday daytime games. The RedZone channels is also available for a fee.
TNTOT provides free live alternate camera angles from select NBA games televised on TNT, including playoff games. It is available on computer web browsers or through the NBA Gametime app for iPhone, iPad, and Android. Get the schedule of upcoming games here.
The PGA Tour produces a lot of free live content from big events that is only available online. This includes following featured groups through their round, coverage from featured holes, and more. It is available through the PGA Tour app for iPhone, iPad, and Android or from a computer web browser.
Watch college sports online
College sports from several big conferences are available to stream through one of the services mentioned above, while others are only available on conference networks through a cable package. There are also a few conferences that have options to watch sports online free or for a fee.
A-10 basketball is available as an online subscription. There are several packages available: $7/24hrs, $15/month, $99/year. Get more info here.
Several Mountain West events are available free through Campus Insiders, including football and basketball. See the schedule and watch live streams on a mobile or desktop web browser here.
Wrap-up and antennas
As you can see, there are several free and paid options to watch sports online without cable. A nice complement to streaming is an antenna, which has ABC, CBS, FOX, NBC, and maybe some local sports depending on location - all free in HD. I highly recommend an antenna to sports fans without cable as long as the reception is satisfactory in your area.
I have always been a big sports fan but have rarely had cable. That's ok with me because the NFL is my favorite sport and is widely available with an antenna. Several other big events I'd watch (playoffs, Olympics, etc) are also available with an antenna. I also live in a different area than my favorite teams, so I wouldn't be able to watch many more of their games by getting cable. There are more options to watch sports online than ever, making cable less and less of a necessity for the sports fan.