The modern day reproductions of the pendants are usually based off of actual archeological finds like this. This design is also found on several runestones and is most likely the way people pictured Mjölnir. Realistically it would have been an impractical weapons, unwieldly and with a very limited range.

There is no such thing as a Norse war hammer. Hammers as such were uncommon as weapons during the viking age, they're a late medieval weapon well suited to beating the living daylight out of a plate armored warrior against whom edge and piercing weapons are rather ineffective.

Its important to note that Mjolnir was NOT a "warhammer." It was modeled after a blacksmith's hammer. Norsemen (although that is a very broad term) didn't use warhammers until the renaissance.

This is the swedish wikipedia that has pictures of archaeological finds of pedants from the 5th to the 8th century in Sweden and as you can see they differ a lot. Runic stones from that period was very diffuse so it is quite hard to really tell what they imagined but the pendants from the period it possibly closest to the real answer.

The oldest recording of the Mjöllnir is about the 4th century on the Sö 111 runestone. Basically a giant upside-down T.

In the 12th century there is the DR 48 Runestone that looks slightly different, but relatively the same.

Bonus: In English Sö 111 reads "Helgi and Freygeirr and Þorgautr raised the rune-decorated landmark in memory of Þjóðmundr, their father."

If you read the mythological story of how Thor got Mjolnir, you'd be made aware that the design of the hammer was due to a mistake made by the blacksmiths who forged it, caused by Loki. As I recall, it was supposed to be an axe.

http://en.wikipedia.org/wiki/Mj%C3%B6lnir

The depictions of Mjolnir must be based on the hammer of a blacksmith, as others have pointed out.