These scripts can be almost as powerful as add-ons, and they're usually just as easy to install. Find something you like at Greasy Fork, click the link to read more, and if you like the sound of it, click "Install this script". A warning dialog explains what you're about to do, so there's no risk of silent installations. There's an option to view the script source, but otherwise you can install it with a click. Installed scripts can then be accessed via an extra tab on the Addons page, where you'll find buttons to set options, disable or remove them.
Greasemonkey also provides simple tools to create and manage your own scripts (click the Greasemonkey button on the address bar and choose "New User Script"). Even experts may find it a challenge to get everything working, but fortunately Greasemonkey has a comprehensive wiki to offer help and advice.
Version 3.2 changes (News):
- The GM_openInTab() API officially supports an open_in_background argument, and always respects user configuration when not specified.
- The GM_xmlhttpRequest() API is now compatible with responseType: 'arraybuffer'.
- Script updates always check the server, never using the browser cache.
- Now @match rules can be customized in script settings.
- Various bug fixes.