Part of it's appeal to literature, is it's age. It's one of the oldest known surviving poems. Part of it is it's vikings, magic, dragons, and that doom laden feel that vikings were known for.
For those who don't know about Beowulf, he is a viking warrior who in his travels, comes across a monstrous entity known as Grendel. The two battle and Beowulf defeats the monster as told in the free version like so: "Then he advances towards Beowulf. A fierce and desperate hand-to-hand struggle ensues. No arms are used, both combatants trusting to strength and hand-grip. Beowulf tears Grendel’s shoulder from its socket, and the monster retreats to his den, howling and yelling with agony and fury. The wound is fatal."
Not satisfied there, Beowulf then goes after said Grendel's mother. He dispatches Grendel's mother with the sword of a giant that he finds in her lair. It already sounds like a Dungeons and Dragons campaign!
After many years of peace, Beowulf's land falls under dragon attack and Beowulf manages to claim victory in this battle as well, but in his advanced years, he is mortally wounded.
The last version of Beowulf I read, and it's been many years, was by Seamus Heaney.
But there is another one that seems to have gained some steam lately that I'm tempted to check out and that's by one of the founding fathers of modern day fantasy, J. R. R. Tolkien.
If you wanted an idea of where Tolkien came up with some of the action for his riders of Rohan, look no further. If you wanted an idea of how Smaug was so mighty and powerful, look no further.
There was also an GCI version that wasn't bad and took some liberties to bring it's own version of the story to modern audiences.
If you're into Dungeons and Dragons, Rolemaster, or any of the fantasy era games, Beowulf should be on your radar.