(Mentha pulegium)
As you may guess from the botanical name this is a member of the Mint family - its small grey-green leaves give off a strong minty fragrance with attractive small lilac flowers atop the stems in summer. This mint loves damp places and is an effective ground cover with flower stems reaching only 30cm high. Likes full sun - partial shade but cannot tolerate dryness in any location.
It is known (proved scientifically!) and is safe to be used as an insect deterrent by being rubbed on the skin to deter mosquitoes & sandflies. Strew the cut herb in cupboards, in dog kennels & bedding and across doorways to deter ants and fleas - planting near a dog kennel can help with deterring fleas from the kennel/bedding.
While it can be used medicinally for various internal issues (similar to any of the mints) it is often discouraged as consuming large doses internally of Pennyroyal can cause serious health issues (coma!) as well as being a known abortifacient (WARNING: avoid taking internally when pregnant).