If you've been waiting for an opportune time to switch to AT&T, it's now. AT&T is currently offering a limited-time sale on a handful of on-contract devices, some of which are pretty desirable. It doesn't cover everything (the almost brand-new Galaxy Note 3 is notably absent, for example) but you should be able to save a pretty penny on your purchase.
The best deal of the lot is probably the LG G2, at least if you're looking for cheap, powerful hardware. You can get LG's Snapdragon 800-powered flagship for just $49.99. Sticking with the flagships, the Galaxy S4 is $99.99 for the 16GB model (with the tougher Galaxy S4 Active going for the same price), the HTC One is $74.99 for 32GB, and the Moto X is $49.99... though you might want to forego the savings on that phone and use the Moto Maker customization site. Shutterbugs can pick up a Galaxy S4 Zoom for $99.99, and fans of ginormous screens can get a Galaxy Mega for $74.99 or a Galaxy Note II for $99.99. There are a few more deals to be had on less popular phones as well.
Those whose contracts have expired may not be eligible for the discounts. Shipping for all the discounted phones is free. There's no mention of a promotional period or expiration date on the site, so if you want in, you should move fast. The prices show as web exclusives, so walking into an AT&T retail store might not yield the same results. Also note that there may be limited stock at these discounted prices - the half-off HTC One Mini is already showing as "out of stock" on AT&T's website.
Source: AT&T Store