The one I've found that's worked best for me is a Chrome extension called Delayed Gratification that actually only delays my access to the site for 30 seconds. It's enough that I can convince myself to sit there and wait for 30 seconds rather than turn it off when I really want access to the site and feel it's justified.

But 30 seconds is also a decent amount of time to convince myself that I don't really need to see that site right now, and often I'll just close the tab a few seconds in and get on with work.

Cue someone writing a "get get-shit-done" script that turns on all the productivity scripts you have at a specified time

Self-control only goes so far, but if (like me) you have trained yourself to flick to reddit as soon as you get even slightly bored, it's a good way to untrain that behaviour.