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Amazon S3 is storage for the Internet. It is designed to make web-scale computing easier for developers.

This service allows you to link directly to files at a cost of 15 cents per GB of storage, and 20 cents per GB transfer.

Photobucket's free account has a storage limit and a download bandwidth limit of 10 GB per month. There's no bandwidth limit on the $25 Pro account.

Flickr offers a free basic account with limited upload bandwidth and limited storage. Download bandwidth is unlimited. Upgrading to a paid Pro account for $25/year removes all upload and storage restrictions. Flickr's terms of use warn that "professional or corporate uses of Flickr are prohibited", and all external images require a link back to Flickr.

ImageShack offers free, unlimited storage with a 100 MB per hour bandwidth limit for each image.