Analysts predict this year's holiday season, which traditionally starts the day after Thanksgiving and lasts till a few days before Christmas, to be the busiest yet for online shopping.

And online shopping will also represent the fastest-growing retail sector this holiday season, predicts the research and marketing firm Retail Forward. It estimates that online holiday sales will go up 16 percent from last year, to $27.3 billion. About one in five holiday gifts is purchased online, according to various industry studies.

One huge new factor in this year's expected online sales boom: free shipping.

According to a joint survey by Shop.org and Bizrate Research, 79 percent of online retailers will offer free shipping for 2005 holiday orders.

Although free shipping seems to be the big trend this year, with many sites that is for basic-level land service. As the shopping season gets under way, that sort of delivery service can easily stretch to a week or more (and even longer to Hawai'i).

Many sites will offer you second-day or overnight express shipping. But costs for those upgraded forms of shipping usually more than negate any savings you get on your purchase. So the earlier you shop the better, if you want to avoid sellouts, and certainly by Dec. 15 for delivery by Christmas Eve (on the Mainland).

One final note: Although online shopping is safe and secure, it's as prone to problems and irritations as mall shopping.

Two-thirds of online shoppers say they've had frustrating online shopping experiences in the past six months, according to the Retail Forward research outfit.

There may not be surly sales clerks or overcrowded stores, but common online shopper complaints include a barrage of those annoying pop-up ads, slow-loading Web pages and being forced to register to make a purchase.