I had long suspected a resource like this existed, but I only recently stumbled across Public Domain Super Heroes. It is a wiki devoted to those super characters that have fallen into the public domain and are free for anyone to use.
Since there is nothing new under the sun, this can be very beneficial. While I've personally tried to create original heroes and villains for my universe, that isn't always an easy task. So it is nice to know that there are characters available to use as a jumping-off point to get the creativity flowing. The downside is that you can only claim the rights to YOUR version, and if the trademark isn't available, you have to do handstands to try to find one that is.
I question some of the conclusions of this site regarding certain characters, and there are many of them (Captain Marvel? Plastic Man?) I would seriously suggest aspiring public domain hero writers avoid. There's also the fact that some characters, like the Nedor heroes and the Golden Age Daredevil, have been used ad nauseaum by seemingly every party wanting some old school costumed crusaders. But even with that, there are still potentially interesting and obscure heroes like Green Turtle and Sky Wizard out there.
As for me, the PD heroes are useful in that they allow me to explore pop culture heroes in my fictional setting. I obviously can't use Superman, Batman, or Spider-Man in a comic story, and would shy away from referring to them in prose, too. But a public domain character like Cat-Man lets me to explore a comic book, movie, or TV superhero without having to create yet another new one. That is really cool. I haven't quite settled on my distinctive versions of Cat-Man and his sidekick Kitten, or whether I'll namecheck more of these antique characters, but it is fun to read about them and learn about the history of comics besides the "Big Two".