In the vein of the DNR guys:
ImmutableAttribute to continuously check that a class or a structure is immutable.
PureAttribute to continuously check that a method is pure, i.e it doesn't provoke any side effect by modifying some fields states.
default C# Keyword in Generic Code. Will return null for reference types and zero for numeric value types
These are things that i haven't used in the past when i should have, but not any more! :)