Be Productive — a barebones Chrome extension

background.js

var urlsToBlock = [
    // matches manifest.json
    "*://*.reddit.com/*"
];

function handleRequest(details) {
    return {cancel: true};
}

chrome.webRequest.onBeforeRequest.addListener(handleRequest, {"urls": urlsToBlock}, ["blocking"]);

manifest.json

{
    "manifest_version": 2,
    "name": "Be Productive",
    "author": "Jacob Bandes-Storch",
    "version": "0.1",
    "description": "Block Reddit and other such generally unproductive things.",
    "permissions": [
        "webRequest",
        "webRequestBlocking",
        "*://*.reddit.com/*"
    ],
    "background": {
        "scripts": ["background.js"]
    }
}