Best Web-only offers - Verizon
All of these carriers have specials for only the Web
Verizon has a sizable selection of products, from phones to accessories, some of which look past their prime, with free 2-day shipping and instant online discounts when you buy online. Also added bonus, the list offers certified pre-owned (read: used) refurbished phones.
For example, they have a refurbished iPhone 4 32GB model, normally $299, for $249. You can also get some refurbished BlackBerry and Android phones for free, one cent or very low prices. You can even get a Samsung Galaxy Tab for $149, quite a savings off the $399 it usually commands.
AT&T has a sizable collection of Web specials, some of them good (the BlackBerry Torch 9800 for 1 cent) and some no deal at all (refurbished iPhone for the same price as a brand new one).
Sprint's collection isn't all that impressive with the exception of the BlackBerry Style 9670, one of the few flip phones Research in Motion has made.
T-Mobile has had Web specials but they are not constant. They come and go and you have to keep an eye on their site. With none running right now, we can't evaluate it.