Some of the most powerful heroes in the comic book world didn't even come from our planet, but thankfully they decided to be friendly. Here are some really cool heroes who are a long way from home:
A Kree warrior who had a long, hard life before finally passing on to the other side, Mar-Vell was very powerful in the 70's. The Nega Bands allowed him to shoot energy beams from his hands. His cosmic awareness helped him to know what was going on everywhere at all times. He could fly in outer space with no ill effects, and faced down big baddies like Thanos and the Skrulls.
No mere villain could take him down (though Nitro had a big part to play in it). Instead, he died of cancer in Marvel's first graphic novel. He came back in a way during the Secret Invasion, though he was actually just another Skrull. The real deal remains one of Marvel's few "he's dead for real" characters.
It's hard to really classify this as an alien, though he is from Thanagar in a way.
He's been killed more often than most major heroes, and he remains in the game. Even though he's currently a Black Lantern, I have a feeling that's only a temporary setback. He always has a way of coming back into the game.
Hawkman will soon be making an appearance on TV's "Smallville", and from the pictures I've seen he looks incredibly true to the character. Now we'll see if he has the same problems with Green Arrow that he does in the comics.
Originally from Zenn-La, Silver Surfer now counts Earth among his allies. He's fought Galactus (and lost...but who hasn't?), and was instrumental in stopping Annihilus a few years back.
Fueled by Power Cosmic, there's not much this guy can't do. Ironically enough, one of the best stories to feature him in recent years is "Silver Surfer: Requiem". It's a great story about the death of the Silver Surfer. If you haven't read it, it's worth picking up.
Currently, he's back working for the big purple dude, but that probably won't stick for long.
Ok, how could I make a list like this and not include the Man of Steel?
The first inhabit from Krypton we met, he changed the face of comic book characters forever. His popularity has grown and waned over the years (and he hasn't had luck with movie appearances in recent years) but he still remains one of DC's most popular characters. Even people who don't read comic books recognize him instantly.
Near limitless power and tough as nails...yet he still gets his hienie handed to him every time he and Batman throw down. Go figure.