These are the most important (most used) variables. To know more details, check out ScreenRenderer.java populateBasicContext and populateContextForRequest methods.
parameters (of course)
availableLocales
availableTimeZones
delegator
dispatcher
security
locale
userLogin
webSiteId
autoUserLogin
person
partyGroup
timeZone
request
response
session
application
javaScriptEnabled
sessionAttributes
requestAttributes
requestParameters