Check it out. It's free and looks cool. I was frustrated by googling free rails hosting all day. Of course there're many hosting sites with shining options. But for testing, it's too expensive. Anyway I found the heroku.com. With Rails2.0 and Amazon S3, this site will dominate all the other hosting sites. Why? because they'll support Gems & Plugins Manager which made me annoying.
There're also some screencasts.
So, for now you'd better freeze your gems by using Dr Nic's Gems on Rails.
To install the plugin:
> gem install gemsonrails > cd rails-app-folder > gemsonrails
rake gems:freeze GEM=gem_to_freeze rake gems:link GEM=gem_to_link rake gems:unfreeze GEM=gem_to_unfreeze_or_unlink
gem install gemsonrails cd app-folder gemsonrails rake gems:freeze GEM=ruby-net-ldap
(Though freezed gem does not work properly now)