All XPath 1.0 functions: starts-with(), number(), count(), string-length(), etc.
Spec defined extension functions:
boolean boolean-from-string(string): true if the argument is "true" or "1", false if the argument is "false" or "0"; otherwise an exception is thrown
string if(boolean, string, string): ternary (?:) operator
number avg(node-set): arithmetic mean
number min(node-set): smallest numeric value, or NaN
number max(node-set): largest numeric value, or NaN
number count-non-empty(node-set): number of non-empty nodes in the argument
index(string): the current 1-based position of the repeat index for the identified repeat element
string property(string): XForms property
string now(): current time
number days-from-date(string): number of days since January 1, 1970
number seconds-from-dateTime(string): number of seconds since midnight, January 1, 1970
number seconds(string): number of seconds in a duration
number months(string): number of months in a duration
node-set instance(string): instance data by ID
User and vendor defined extension functions