For Chrome users:

1) Install http://chrome.google.com/webstore/detail/xat-tools/ogcdcefmgcofkjjckhpjnhopehbhjmef and click on the xat icon that appears on the toolbar. 

2) Click the gear icon in the window that appears.

3) Scroll down to the bottom and click on "Advanced Mode" to enable the advanced mode. 

4) Go back up to the top of the screen and click the checkbox next to "Bypass linkvalidator".

5) Make sure the drop down menu beneath this is set to "Always" instead of "Whitelist only".

6) Reload all your xat chats (if you have any open) and now all links should work!

Note: This is for Chrome users only there is a Firefox version but it is not as good at http://addons.mozilla.org/en-US/firefox/addon/xatalert-tools/

The Firefox version seems to work, but you have to individually add all the sites you want to bypass linkvalidator for. So if you just want YouTube or something this might still be useful.