I've recently started using Greasemonkey. This is an add-on for Firefox which can alter the functionality of websites using specially-written scripts. You can read more about it here.
There are lots of scripts available written by other people, but I generally just use scripts I've written myself.
You have to install the Greasemonkey add-on for the following scripts to work!
They also appear to work with Tampermonkey for Chrome.
World Of Spectrum
The World Of Spectrum website is really great if you're a fan of the Sinclair ZX Spectrum. Unfortunately the site is broken in quite a few ways.
The site links to images of pages from old computer magazines. Sadly, all the links are broken. The following script repairs the links by pointing them to a mirror site instead.
Also, some of the in-game screenshot images are broken as well (but weirdly, not all). The following script fixes the broken images.
Um. That's it for now!