With the exception of a few sports, this is the world of fast action photography. This means fast shutter speeds, fast "emulsions" (ISO's), fast continuous shooting rates, and fast lenses. The first two, fast shutter speeds and high ISO's, are available in most prosumer d-SLR's and compact cameras, so you should have no problem.
The last two, fast shooting rates and fast lenses, are not widely available. Many mid-level d-SLR's can deliver only around three frames per second (fps), and burst rates of around 10 to 20 images. If you intend to shoot fast action regularly, better invest in a camera with fast shooting rates and burst rates.
Neither are long fast lenses (f12 or f/2.8) within reach of the average photographer, who probably can't afford anything faster than an f/3.5 or thereabouts zoom lens.
Unless you have only one zoom and a prosumer d-SLR, you will need to pack only the most essential gear and leave the others behind. Showing off a fisheye at the press section will get you more sniggers than envious glances from fellow photographers. Different sporting events require different lenses, for sure, but for most events, you will need a long (moderately long or very long) zoom lens and a moderate to extreme wideangle zoom. That nifty 80-200mm zoom will suddenly look underwhelming when you're shooting from the bleachers.
Be aware you may not have the opportunity to switch lenses during the event. Either the sport may be too fast and furious for changing lenses, or the venue too dusty to be exposing a camera sensor to.
You can bring one, but be prepared to leave it in your bag the whole day. Flash photography is not allowed at most sporting events, as it can distract players and ruin concentration.
Bring enough to carry you through the whole sporting event, if you think you will be taking a lot of pictures, bring a portable hard disk into which you can empty your cards during intermissions.
Tripod or Monopod
Don't delude yourself into thinking you won't need one, since you'll be favoring fast shutter speeds anyway. You'll need one if you shoot indoors under artificial light or with long, heavy lenses. Come to think of it, it's not the slow shutter speeds that will dictate whether you bring a monopod or not. If you don't want your arms to age a hundred years from supporting a blunderbuss lens the whole day, bring a monopod.
During a game, you can't afford to turn off your camera for a minute, not even to put it on standby mode. The slight delay in turning on a camera or waking it up can make you lose a picture. So bring spare batteries, and don't wait till the battery in your camera nears empty; take advantage of a lull in the action, or an intermission, to switch batteries. Intermissions such as the minute between boxing rounds are also good times to swap out memory cards.
These are not photo equipment, but should be part of any sports or other photographer's gear. Remember that you may be out in the open for hours on end, a hat will keep you from getting a minor sunstroke, not to mention protect your eyes from direct sunlight. Also, bring a bottle of water to rehydrate you in the field. Finally, don't forget the other extreme, a sudden downpour, so remember to pack a raincoat for yourself and a large plastic bag (such as a trash bag) for your gear.
Barry Wong, "In The Bag: Food For Thought", Sportsshooter.com