Stuffed blocks websites on OSX. Think a command line version of Self-Control.
gem install stuffed
stuffed add reddit.com - adds reddit.com & www.reddit.com to the blocked list.
stuffed remove reddit.com - removes reddit.com & www.reddit.com from the blocked list.
stuffed list - lists all sites being blocked.
stuffed off - toggles blocking off.
stuffed on - toggles blocking on.
Stuffed edits your /etc/hosts file so most of the time you'll need to run sudo stuffed <task>.
There is some OSX specific code to flush the DNS cache, I haven't tried or tested any other OS's.
If you run into issues, there was a backup of the hosts file created at /etc/hosts.backup.