Global variables in Javascript

L_Menu_BaseUrl - the base URL of the site.

L_Menu_LCID - the LCID of the site you're in. Useful if you need to determine the language of the site.

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L_Menu_SiteTheme - the name of the theme applied to the site.

There is one more useful variable, but this one can't be used on custom master pages that you created. This one is used in the SharePoint's default pages:

_spUserId - the ID of the logged in user