This site is delightful, you want something, it has it. Image galleries, ajax form, form validators, etc. All scripts are written for every popular library like mootool and jquery. The scripts are handpicked and the quality is there. I don’t think you’ll find a poorly written script. My scripts never got in this site actually !
A newcomer from the noupe.com people. This website is similar to ajaxrain, a little easier for scripts to get in, but you got a vote feature to determine if the script is good or not and it got good search options. There is a lot of ressources on this site, and it’s likely you’ll find what you’re looking for. In fact you may find cool things you’re not even looking for!
From what I know, it is one of the oldest good snippets library. Not updated often, and no search options but there is some jewels if you take time to digg it.
Perhaps a lesser known website but there’s some good examples. Unfortunately there’s no search by library like jquery. Which makes it a bit tedious to find what you want.
A friend of mine. He got some really good scripts, it is worth checking out if you want a jquery lightbox or popin.