List of default Task Sequence Variables
Here is a handy link to the Technet library page that contains all of the default Task Sequence Variables. Bookmark this one to keep it handy!
http://technet.microsoft.com/en-us/library/bb632442.aspx
It has recently gone through an update, so most of the variables are no longer on the same page - but the links are on this page to take you to the specific sections.