I've been working with a saline (copper) sulfate etching solution on steel plates recently - I was surprised by how well it works considering I was introduced to it as a zinc etch. You can etch steel and zinc in the same bath (at the same time!) with no adverse results to either metal. It's also more environmentally sound than traditional acids, with less fumes, and very reusable.
Two new plates I etched with the saline solution:
"Birdcage Study"
Steel plate etching
6" x 4"
"Sisters"
Steel plate etching