There are many sites you can use. Some are chargable while others are free. Different people have different requirement & preference, you may want to do a search on Google to see which are the sites available and suitable for you.
There's a thread with a poll (but it's somewhat outdated). I use SmugMug (paid) because I like the way my photos are presented and the people running the site. Many use PBase (paid). Free sites include Picasa and Photobucket. I'd stay away from those photo printing + sharing sites (Shutterfly, SnapFish, Kodak) and anything that requires your viewers to register before they can view your photo.