I would recommend Lua.  
[Python][1] is very popular, too. Many featured game engines (Blender, for example) use it.  
C++::Boost has a [library][2] to work with Python.  
I've read about [Squirell][3], but didn't use it.  

You can read [this Game Engine overview][4]. There is a `Scripting` column. You can see that Lua and Python are most popular scripting languages.


  [1]: http://www.python.org/
  [2]: http://www.boost.org/doc/libs/1_44_0/libs/python/doc/index.html
  [3]: http://squirrel-lang.org
  [4]: http://gpwiki.org/index.php/Game_Engines