These are the most important (most used) variables. To have more details, check out ScreenRenderer.java populateBasicContext and populateContextForRequest methods.
This means that you don't have to worry about these variables : they are available from the 1st line of your scripts (BeanShell or Groovy)
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