Hundreds of scripts, for a wide variety of popular sites, are already available at http://userscripts.org.
You can write your own scripts, too. Check out http://wiki.greasespot.net/ to get started.
You can install scripts by right-clicking on links to them and selecting "Install User Script...". Or, you can select the same option under the tools menu while viewing the contents of a user script file.
Each user script will be automatically included with one or more web pages. You can fine-tune which pages will be affected when you install or by going to Tools > Manage User Scripts.
To download script you can go here http://userscripts.org/