Don't like the way a site looks or want to add some code to enhance it? Tomado is a simple webapp for modding a site to your liking.
Once you're satisfied with your mod, publish it to get a custom URL for you and anyone to access the redesigned site. There's a cool one for xkcd, for example, that lets you swipe to go through the comics at xkcd.tomodo.me. Another mod in the gallery modernises Hacker News. You can also view the source code for any published Tomodo sites to see how it was done.
Tomodo is like Greasemonkey, except it works on all browsers and mobile devices — and doesn't require you to install a browser extension. It serves as a proxy between the browser and the original site, without scraping or saving any of the data to servers. It's fun to play around with too.