AdiIRC

As of, Friday 27th April 2018 at 15:06 PDT Some of these are no longer valid.

http://www.hawkee.com/mirc/snippets/
http://www.mircscripts.org This domain expired 4-21-2018 (New replacement site - mircscripts.net)
http://www.tg007.net
http://mircscripting.info/forum/
http://forum.swiftirc.net/viewforum.php?f=34
-http://sephiroth.bounceme.net/board/viewforum.php?f=4 This site cannot be reached
http://www.forunsbb.com/tutoriais-mirc-scripting-f82.html 404 Error
http://www.xise.nl/mirc/
https://mypastebin.com/search.php?page=1&search=*&cat=description&by=DESC&results=300

Updated - 18 Jan, 2023 Of this list, only the following 3 are still active.

http://www.tg007.net
http://www.hawkee.com/mirc/snippets/
http://www.xise.nl/mirc/

There is 1 more that is new and is active related to mSL and is almost compatitable with AdiIRC too.

http://mircscripts.net

The http://mircscripts.net has expired, and has moved to https://mircscripts.info/?page=home
As for script compatibility, the newer your $builddate the more likely that scripts will work. A lot can depend on what the script is trying to do. For example some scripts use $readini($mircini,section,item) to check for settings, and would need different ways of making equivalent code.

Another difference can be very old scripts that would use obsolete syntax where identifier parameters are outside of their parenthesis, like:

//echo -a $read -ntl1 $qt($mircini)

... where you should instead use the modern form like $read($qt($mircini),nt,1)