Most used seems to be pbase and smugmug. I looked at both in January and picked smugmug.
Pbase limits the number of images you can keep (about 2000 I think) but no limit on bandwidth, while smugmug limits bandwidth (4GB per month) but no limit on images. I don't post many photos except here and in May I only used .5GB so the limit does not seem critical.
One thing with smugmug that I really like is that I use iView for image cataloging and add captions with it to the images. When I upload the images to smugmug, they find the caption and display it with the image so I don't have to do anything more to describe them.
I believe that smugmug has the reputation to be a little more user friendly and quicker to respond to customer questions or problems than pbase.
You can save $5 on the smugmug by using a code or email from a current user. That user also gets a credit when you do it. I used Frank Wilson (Flew) since he is one of the founders of Nikon Cafe. If you choose smugmug I suggest you do the same for either Frank or Patrick, our founders.
Just a positive note for Pbase (I use 'em and love 'em):
They just did an update for members yesterday. For the basic account it is now 300MB for the same $23 that's a 100MB increase. Every year for the past 2 years they have added 100MB to the basic account and kept the price the same.
Also, Pbase does not limit how many images you upload. You can pack as many images you want into your 300MB (I usually set my online images to 1024x768 and the file size is around 200kb, just compressed enough so they look pretty )