I vote Dropbox for sharing photos. There was one cousin whose photos I was really keen to get so I just gave her the password to my Dropbox to upload the photos to.
I reckon just set up a dropbox account with generic login/password, then give everyone the account details so they can log in and upload from home. the website is really very easy to use - once you log in you just have to drag your folder of photos from the desktop/documents folder into the webpage in the browser itself and it starts uploading them automatically.
The trick is reminding/getting people to upload their photos. I suspect you might have more luck emailing the upload instructions/reminders to your guests say 2 weeks after the wedding than relying on little business cards at the reception that they might just drop into their bags and forget about.