It's a Nepeta. It is in the mint family (Lamiaceae) but not in the actual Mentha genus.
The flowers and leaves look right for Nepeta racemosa 'Walker's Low' and it is a commonly available cultivar, so that's a good possibility. However, it would be hard to say for sure because some varieties look similar.
Catmint (Nepeta x faassenii 'Walker's Low')
