The Washington Post has a helpful up-to-date chart on current airline baggage policies.
Southwest stil doesn’t charge for the first or second checked bag. Airtran doesn’t charge for the first, and discounts the second if you prepay online. Delta, and Continental don’t charge for the first, but do for the second.
Personally I expect that Delta and Continental will eventually join the leaders. Unless they see significant bookings coming their way driven by the lack of fees — and the fees are sufficiently conoluted for most passengers to make sense of so I doubt it — they’re walking away from revenue if they don’t. I do see the opportunity for a clever ad campaign to differentiate their products, but in the end all the evidence I’ve ever seen on coach travel is that the driver of airline selection is price, nearly to the exclusion of everything else, so it’s a question mark whether such a campaign could ever succeed.
Elite status and flying in a premium cabin generally exempt flyers from these fees. The trick here is that any status gets you what you need, including alliance partner status. And companions on the same reservation usually get the waiver as well. United is the most generous: gifted Premier Associates benefit from the waiver, and all passengers on the same reservation benefit from that member’s status.