Variables always available in screen context

These are the most important (most used) variables. To have more details, check out ScreenRenderer.java populateBasicContext and populateContextForRequest methods.

parameters (of course)
globalcontext

delegator
dispatcher

security
locale
userLogin

componentName
webappName

webSiteId (only for webapps where it makes sense and are defined in their web.xml file, like ecommerce)
autoUserLogin
person
partyGroup
timeZone

request
response
session
application

javaScriptEnabled

sessionAttributes
requestAttributes
requestParameters

eventMessageList
errorMessageList
nowTimestamp

externalKeyParam (it contains [externalLoginKey=${externalLoginKeyValue}] or null if security.login.externalLoginKey.enabled is set to false)