The legacy carriers -- American Airlines, Continental Airlines, Delta Air Lines, Northwest Airlines, United Airlines, US Airways -- all have very similar luggage rules. The rules are typically:
Checked baggage allowance
- Two bags at up to 50 pounds each
- Maximum is 62 linear inches (length + width + height) per bag
Excess baggage weight fee for any bag exceeding 50 pounds
- Between 51 and 70 pounds - $25 per piece
- Between 71 and 100 pounds - $50-80 per piece (rate varies by airline)
There are also fees for excess size. And beyond a certain size, the airline will refuse the luggage entirely.
Carry-on allowance
- One carry-on bag
- Must fit in an overhead compartment or under the seat
- Maximum is 45 linear inches (length + width + height)
- Should not weigh more than 40 lbs/18 kgs
- One personal item, such as a purse, briefcase, small tote bag, or laptop computer
In addition, passengers may bring on loose items such as coats, newspapers, and "to go" meals.
Assistive devices for passengers -- such as wheelchairs, walkers and CPAP machines -- do not count against the allowance.
Of course, it's a good idea to check your carrier's website for your carrier's exact rules. For very detailed information, it's best to look at your airline's Contract of Carriage.
The so-called low cost carriers (LCCs) each have their own rules. Some are just like the legacy carriers. Some are more generous than the legacy carriers. For example, Southwest allows three checked bags. JetBlue used to allow bags up to 70 pounds, but their "free" limit has now dropped to 50 pounds per bag.
The short answer to the OP is that Spirit probably has (or will soon have) the most restrictive rules, but I'm certainly not aware of the exact rules of every LCC, regional carrier, commuter/feeder airline, or international carrier.