FlickrFlickr, currently own by Yahoo!, is perhaps the most popular photo sharing site in the world. As of late 2009, it reportedly hosted over 4 billion images. One of the main reasons it is so popular is that many bloggers use it to host images, which they embed on their blogs. According to Alexa.com, Flickr is the 21st most popular website in the US, based on traffic.
On Flickr, you can get a free account where you're allowed to upload up to 100 MB of images per month in addition to two videos. Although free users can upload 100 MB of images per month, they will only be able to see the most recent 200 photos in their photostream, which many might view as a negative. Also, if you have a free account, you need to be sure to keep it active. Accounts that are inactive for 90 days are subject to deletion.
If you prefer to pay for the service, you can get a pro account and upload an unlimited number of images and receive unlimited storage. This may only be recommended for very serious digital photographers, due to the vast amount of free photo resources and storage sites on the internet.
ShutterflyShutterfly, another very popular photo sharing site, is not quite as popular as Flickr. According to Alexa.com, Shutterfly ranks 460 in the US, based on traffic.
Despite the difference in traffic rank, Shutterfly has a few features that might make it more appealing than Flickr. The biggest difference from Flickr is that Shutterfly allows you to upload unlimited images at no cost. In addition, they claim to never delete photos. One reason Shutterfly may not be as popular as Flickr is that their site is not as visually appealing (although it's just as functional). Also, Flickr is backed by the Yahoo! brand, whereas Shutterfly stands on its own.
Shutterfly offers some interesting features besides photo hosting, which include the ability to turn your photos into photo books, cards & stationary, and personalized websites. As expected, these are premium features.