Are there any scripts to integrate social networking site's chat and posting features into a webpage using PHP or JS... for example a section with Facebook chat, a section to post to twitter and maybe even YouTube comments.
This depends on the social network. Facebook's developer center will help you. http://developers.facebook.com/
There is 2 social application from Facebook that can be used as chat/comments box.
- Comments - This is a basic comment box where people will be able to add comments.
- Live Stream - This basically acts as a chat box where people will be able to talk live.
Yioiu should investigate twitter Anywhere -
http://dev.twitter.com/anywhere/ a cool implementation of what you seek with a few lines of JS
Update: Twitter's Anywhere project is now deprecated and ceased functioning on March 5th 2013.
Here's some options for Google:
But... just because you can doesn't mean you should. Think about this in terms of value to your site's visitors. Don't do it just to be cool.
UPDATE: Google has retired Google Friend Connect in favor for Google Follows Blog which uses Google Plus. Additionally Google Buzz has also been retired.