I use Leechblock on Firefox (including mobile) [1]. It's also available on Chrome [2]. You can set delays, hard block, set custom time ranges (i.e. only block during working hours), make it overridable, and a lot more.
I really dig it, it helped me stop using Reddit :^)
Does this let you set a timer like described in the comic? I can't see it. I'm looking for something that makes me wait a minute before allowing a specified site to load.
It seems like it. Pick a site, choose to block it all day, use blocking method delaying page and set delay time to a selected number of seconds. Seems to work, but haven't tried this configuration before. One has to sit through the delay, if you switch context, the countdown stops(!)
As the other commenter mentioned, that'd be the Delay page. I use 60 seconds for sites like hackernews at work and it really curbs the impulse to waste time while still allowing you to use the site if you really want to.