Ibis the Invincible. In 12th Dynasty Egypt, four thousand years ago, Prince Amentep is imprisoned by his rival, the evil magician the Black Pharaoh. Amentep, or "Ibis" as he is known, is not as powerful a magician as the Black Pharaoh is. But while Ibis is imprisoned a relative gives him the "Ibistick," a magic wand with which the user can control or create any object. Ibis uses this Ibistick to escape from jail and then kill the Black Pharaoh. Unfortunately Ibis' beloved fiance, Princess Taia of Thebes, was wounded during the final fight. She is put under a spell so that she will not die but will sleep for 40 centuries. Ibis, to be with her, puts himself in a trance as well. He awakens in 1940, covered in mummy clothes and residing in an American museum. He goes to Europe, awakens Princess Taia, and then goes on to fight crime and evil. Ibis is a magician, but most of his powers come from the Ibistick, which can initial do almost anything but later becomes powerless against black magic.
First Appearance: Whiz Comics #2 (Fawcett)

Immortal Man. Jakeoster contributes the following:

Chuck Magnon was a Cro-Magnon man who was struck by lightning 100,000 years ago. Instead of killing him, it somehow made him immortal. He’s roamed the world picking up the knowledge of ancient Atlantis, Greece, and Rome. To put all that accumulated knowledge to good use he set up an office as "C. Ro-Magnon, Questions Answered.".

Inferno. Inferno is a fire-breather at a circus. Unfortunately, he's also mixed up in some illegal doin's. Fortunately, Steel Sterling comes to town and sets Inferno straight. Inferno then goes on to fight crime, first as the circus worker, moving from town to town, and then later as a private eye. He has the super-power of breathing fire from his mouth; he can burn his way through steel doors and walls and can melt bullets before they reach him.
First Appearance: Zip Comics #10 (Archie)

Inspector Bancroft. Inspector Bancroft works for Scotland Yard.
First Appearance: Mystery Men #1 (Fox Features)

Inspector Dayton. Dayton is a crime-busting police inspector.
First Appearance: Jumbo Comics #1 (Fiction House)

Invisible Avenger. Ronald Byrd, blessings be upon him, gives us this:

Next, it's back to the present (I guess) for Buzz Allen, the Invisible Avenger.  Buzz is a young ham radio operator whose father, like many another golden age father, is slain by racketeers.  Later, when installing a "super-electron tube" in his radio to boost its power, Buzz accidentally turns his radio invisible.  Rather than market this oddity, he converts it to an invisibility-inducing belt (which would have been even more marketable), and he and a friend, Will Lawrence, set out to do the righting-wrongs thing, starting with the racketeers.  It's known that in #3 they discover the hidden city of Atlantis (boy, THAT was a quick jump in threat-level, huh?) somewhere in the vicinity of Alaska and prevent its leader Phatso (yeah, HE's got a future in politics) from taking over America.

Invisible Hood. Kent Thurston puts on a chemically-treated robe, complete with hood, and fights crime, changing his name from Hooded Justice to Invisible Justice to the Invisible Hood as issues went by. The costume gives him invisibility.
First Appearance: Smash Comics #1 (Quality)

Invisible Justice. See the Invisible Hood.

Invisible Man. One Dr. Gade was in his laboratory working with special chemicals when his traitorous assistant pushed him into a blast furnace. The chemicals spilled all over Dr. Gade and turned him invisible as well as fire-proof. He managed to master his abilities and used them to fight crime. He is able to turn invisible at will, but is dependent on the rays of one of his machines to restore his strength.
First Appearance: Mystic Comics #2 (Timely)

Invisible Scarlet O'Neill. Scarlet, a beautiful redhead, was the daughter of Dr. O'Neil, a scientist who was experimenting on creating various weapons for America. Scarlet accidentally stuck her finger in front of a "weird-looking ray." This gave Scarlet the ability to make herself invisible at will by pressing a nerve on her left wrist. Scarlet decided to use her new power to help America and to do good for people. And so she did, passing up romance on numerous occasions to fight Japanese and German spies, saboteurs, gangsters, and other dobadders.
First Appearance: Famous Funnies #81 (Eastern Color)

Invisible Terror. "When Dr. Cortell, noted scientist, died, he turned over his most startling discovery, a chemical formula for invisibility, to Tom Wade, his young assistant. Tom Wade now uses the formula to become The Invisible Terror, that unseen mysteryman menace of all crimedom!"
First Appearance: C-M-O Comics #1 (Centaur)

Iron Ace. Centuries ago a legendary warrior called "Iron Ace" fought by Charlemagne's side for France. When Iron Ace died Charlemagne predicted that the Iron Ace would return, as "the champion of freedom." During World War Two Captain Britain, a British pilot , is shot down over France and forced to hide in the castle of Dr. LaFarge. LaFarge is the leader of a resistance group, but the Germans shoot him after they track Britain to his castle. Captain Britain puts on the old armor (still in perfect shape) of the Iron Ace and kills the Germans in the castle with the Ace's sword. He then makes himself a plane and, still wearing the Ace's plate armor, takes to the skies and kills heaps o' Germans. The Ace has no superpowers, but like his plane is so heavily armored that he's bulletproof.
First Appearance: Air Fighters Comics #2 (Hillman)

Iron Gates. Gordon "Iron" Gates is "an ardent and wealthy student of scientific crime detection."
First Appearance: Captain Aero #1 (Holyoke)

Iron Munro. Iron Munro is a strapping young lad, a two-fisted adventurer of the future who flies from planet to planet doing good and fighting against evil. He is helped by his friends Spencer Carlisle and Anto Raul. Iron seems to be invulnerable and may have superstrength, but his main powers are clean living and a good right hook.
First Appearance: Army & Navy Comics #1 (Street & Smith)

Iron Skull. Iron Skull was a super-powered android who fought crime and the Axis. He is superstrong, bulletproof, can act as a "human torpedo," but is vulnerable in the "main artery in his left forearm."
First Appearance: Amazing Man Comics #5 (Centaur)

Iron Vic. Quoting Ed Love:

A dying amnesiac found on the beach is treated by Dr. Degnan and Professor  Carvel who had developed a serum that when treated with infra-red rays would grant great  strength and intellect. Carvel dies soon after. His past a mystery, Vic becomes a mystery  man aided by Dr. Degnan. Vic does his bit in fedora and fancy suit and cape but no  mask.