Unused ones are ` and $. However, some are used under very specific situations, such as %, ~, and \ which may still be available for other use cases. We will also free ' once Elixir v2.0 arrives (as single-quotes for charlists are deprecated). However, I don’t think this feature is important enough to justify “spending” $ on it (if the sigil such as $ is a requirement, I’d consider it a deal breaker for now 
).
EDIT: there is also the option of using something like @@foo for this.
