amxmod works with cs 1.3 too but there are very few plugins that (fully) work with it.
and here's a list of mods that should work good with Xash so far:
http://www.hlfx.ru/forum/showthread.php?threadid=3060
besides these ones I would say that these here are good working too:
Code:
- Ricochet
- Opposing Force (didnt play the full game with Xash yet though)
- Deathmatch Classic
- Team Fortress Classic (except the rocket launcher bug I didnt notice anything else)
- pretty much every Half Life Singleplayer Mod that doesnt change much of the original game