The Valkyrie & Einherjar
Both of these entities were made before Ragnarok, by the Goddess Freya. The purpose of them was to create an army worthy of defeated the Hoards of Feigr created by Hel for the battle of Ragnarok. Neither of these entities are made any more, for Freya refuses to do so. Ragnarok is no more, so there is no more need for mortals to become immortal and fight and feast in Valhalla or Folkvangr until the end of days.
The Valkyries
The Valkyries are those who in life, were God-touched: Volur. Freya could see this when the mortal form died, and could claim the soul touched by magic for herself. The process of creating a Valkyrie, to make a mortal immortal, is a closely guarded secret, as is that of creating an Einherjar. The Valkyrie do not remember their mortal life, and knows nothing beyond what they are now. They have the powers to move around the realms and collect the souls of the dead who were worthy to go Valhalla, but they is not all they did. To be a Midgardian Valkyrie was a great honour, but up on Asgard they also performed other tasks for the Aesir. They took messages, helped tend to the Gods and Goddesses, and tended to the warriors feasting in Valhalla. There were many roles a Valkyrie could aspire to, the most coveted being the chance to travel between the realms and collect the souls of the worthy.
Valkyrie have not been made since before Ragnarok. With the battle to end all battles not ending anything, and Midgard shut off to the Higher Realms, Freya sees no reason to continue their creation. The Human's and Alfar who have come to Asgard can, and do, perform many of the tasks the Valkyrie used to perform. Those Valkyrie remaining who survived Ragnarok are sworn only to Freya, and those she allows to use their services.
The Einherjar
The Einherjar, pronounced 'EIN-YAR' are the warriors of Midgard considered worthy enough to join the ranks of Asgard at Valhalla or Folkvangr. Their souls taken by the Valkyrie and Freya makes them immortal, though the process is a closely guarded secret just like that of the Valkyrie. The Einherjar remember their mortal life, and fight for the honour of their decendents. They fight each and every day, and feast each and every night.
This they did until Ragnarok. Those who survived Ragnarok, and managed to return through the Bifrost before it was sealed now reside in Folkvangr with Freya, or as Baldr's Throne guard. Frigg also has a guard of Einherjar, as does Magni, though they do not carry unit names like Baldr's do. Within the Asgardian Army the Einherjar serve as unit commanders of mortal soldiers made up of Human's and the occassional Alfar. They train for the day when Asgard was attacked, for all know it is only a matter of time.