A while ago I published a open source photo gallery built with Ruby on Rails. You can find more info on balderapp.com or on the GitHub repository. I have now updated this to a new version which supports S3 storage and so it can run off Heroku (which has no file storage, just a database). Heroku is a really nice free hosted ruby platform and it is very easy to put up simple apps on it as this blog post will demonstrate. Just sign up for Heroku and S3 and type of a few commands in terminal and you are ready to go!
git clone email@example.com:espen/balder.git
Edit config/balder.rb: uncomment heroku & s3 settings. Insert your S3 credentials.
git commit -a -m 'settings'
sudo gem install heroku
heroku create APPNAME
git push heroku master
heroku rake db:migrate