Pegs are not the same as currency unions but the HKD has been pegged to the USD at around HKD 7.78-7.8 for some 3 decades.

The JOD has been roughly pegged to the USD at ~USD 1.4/JOD

Another peg (to the EUR/FRF) is the French Pacific Franc used in the French pacific possessions.

Back to currency unions, a little-know one is the United Kingdom where you have English Pounds (I'm not using the term British Pounds/GBP because they're issued by the Bank of England) as well as Scottish and Northern Irish ones (never physically seen one of the latter) which have the same identical value. Haven't looked up what Manx and the Channel Islands (boths parts of the U.. but not the EU) use.

Only the the Bank of England-issued bank notes are acceptable outside the U.K. however.