Last week I was installing again rmagick on a Ubuntu server, then I discovered an very easy way to do it.

$ sudo aptitude install librmagick-ruby

Edit 10/20/08: Avoid this tutorial it doesn't work, if you wanna install rmagick use the old fashion way:

$ sudo apt-get install imagemagick

$ sudo apt-get install libmagick9-dev

$ sudo gem install rmagick