CallingMart.com. One of the oldest and best known online airtime sellers, CallingMart sells airtime for over twenty operators. CallingMart's everyday prices include a 1% to 5% discount on purchases over $18 for some providers including Airvoice, H2O, Verizon and GoPhone. The code CA3P-1207 provides an additional 3% off the same operators.
Almost every month, CallingMart has a sale that lasts for a week or two that offers 10% off AT&T GoPhone; 5% Off Verizon, H20 and Airvoice; and 3% off Red Pocket airtime. The sales are usually holiday themed and occur around a holiday.
Last year TracFone increased its wholesale prices on airtime for all the TracFone brands including TracFone, NET10, Simple Mobile, Telcel America and Page Plus. Most online vendors no longer offer discounts on airtime for the TracFone brands and some, including CallingMart, charge a fee, 2% in CallingMart's case, on TracFone brand airtime.
Megasaver.com is one vendor that still discounts Page Plus airtime. Use the promo code 5off for a 5% discount on Page Plus. This is a limited time offer with no set expiration date. H2O refills are also 5% off at Megasaver
PinCheap.com sells airtime for most operators and offers discounts on many. Currently discounts range from 0.5% on TracFone and Page Plus to 10% on AT&T GoPhone.
All the vendors listed above do not collect taxes on airtime. However, in most states your are still liable for uncollected taxes on online purchases and are supposed to report and pay them on your state income tax return.
Target.com and Target Stores have occasional week long 5% to 10% off sales on prepaid airtime. If you purchase using a Target Red Card you get an additional 5% off. Target collects all applicable state and local taxes both online and in store.
I'd like to make this post the go to resource for finding prepaid airtime deals. If you know of a reputable source of discounted airtime that's not listed above please mention it in a comment and I will add it to this post.