
One of the websites I've visited on occasion is a site called "Public Domain Super Heroes", which lists all of the heroes whose copyrights have expired and thus have become part of the public domain. In other words, anyone can use them without having to pay royalties.
Much to my surprise, I found Captain America listed as one of those characters.
http://pdsh.wikia.com/wiki/Captain_AmericaBUT, before you get excited, apparently some very specific limits were put on this.
First, the character's name is Grant Gardner. Anyone else is owned by Marvel.
This Cap uses a gun, not a shield.
There is no super-soldier serum.
There is no Bucky Barnes.
Here's the website for the full listing. It's not really user-friendly, but it's worth a look-see.
http://pdsh.wikia.com/wiki/Public_Domain_Super_Heroes