I liked the idea of the "Strange Costumes of the Batman" back in the day, and thought that carried considerable appeal. I wanted costumes like those to start showing up in the stories. Alas, they were a one-time thing. 

Supergirl's look was in constant flux throughout the late 60's and early 70's as she accepted reader suggestions for what she should wear and must have stocked up on double-sided tape to hold some of them in place. The Legion was also responsive to reader suggestions in this regard, with good and bad results. Saturn Girl's pink bikini is a personal favorite of mine, almost because it's so much the opposite of her previous outfits. But again, these likely fall under the umbrella of permanent costume changes rather than alternative wear the heroes only slip into now and again.

Interestingly, when Lightning Lad, Saturn Girl, and Cosmic Boy first appeared, they wore costumes we did not see again until they showed up on the members of the Legion of Super-Villains. Since the LSH appeared to both Superboy and Supergirl, who have a noticeable number of years between their ages, the writers first explained that Supergirl's group were the children of the originals who visited Superboy. This was quickly set aside as unnecessary, and the dialogue was removed from all future reprints of the story. Yet...

The costumes worn by the group who first visited Superboy go on to appear as those of the LSV, so is it possible perhaps that it was THEY who first went back in time to contact the Boy of Steel and razz him with their initiation pranks? Maybe throw him off the idea of joining when the Legion actually did show up later? It would also clear up the question of who all those members shown in the shadows were in that first story. Just blue-skying. 

There was a fun story illustrated by Don Newton in a Robin back-up story in which he agrees to wear three different outfits designed by students on-campus (actually the readers once again) as he goes out on patrol over the next three nights. One of the them was equipped with a rocket pack. 

Superman once wore a fairly ghastly Sci-fi uniform in yellow and purple with a space helmet, winged boots, and an anti-kryptonite belt for the duration of one story. Similarly, Underdog was made to feel tragically old-fashioned due to his costume, and exchanged it for a patriotically-colored glitter thing with bell-bottom slacks. That also lasted the length of the story that introduced it.

The most prevalent use of alternative costumes is probably Iron Man's array of specially purposed armors. While I agree with those who say it diminishes the versatility and power of the original, base-line suit, I still come down in favor of them because, more importantly, these alternate armors are just plain fun. As a kid, I fell hard for the polished black, red-eyed Stealth Armor and the Space Armor which was introduced around the same time. Later we got the undersea armor, the Hulkbuster suit, and the Godbuster, all of which are just plain fun to have around.

I wonder if Eric might not have been thinking of Cosmic Boy's almost-nude corset ensemble from "The Rokk Krin Horror Picture Show" in his comment.

I don't love a lot of alternate costumes, but, yes, Iron Man's blue/black armor was awesomely cool!

But event that first one - I think it's really busy. I don't get the eagle symbol; I couldn't associate that with anything about Colossal Boy save that he was from Earth and, presumably, from the United States - if it even still exists in the 30th century. And I really hated purple for the harness and belt.

Brian H. - No, I quite remember both the Colossal Boy outfit and the Cosmic Boy/Boss of the Beach costume. :) I really liked that A) Cosmic Boy's design looked GOOD - just a little incomplete, and B) they got him away from pink (ditto for Element Lad.) I have nothing against men wearing pink... I just thought that the color didn't really look good on ANYONE. (I was hoping that Saturn Girl's outfit would stay red, but I guess they like going back to that pink.)

Other discussions... speaking of Robin, shall we discuss his redesigns? Earth-2's Dick Grayson donned a compromise between Batman's outfit and his own in his JSA debut. I thought it was kinda quirky... but not bad. And quite frankly, Robin seemed such a good addition to the JSA that I was happy to see it.

Then Robin-2 was there when Robin-1 debuted in his Neal Adams designed outfit. I enjoyed the heck out of that one. Robin of Earth-2 then got almost the same outfit in All-Star Comics #58 save that it had a cape instead of yellow glider panels. Shame... a gliding Robin would have made sense, and been a nice little change.

One of the Legion's changes was a smart one, I think, and an idea that I liked. Post-Crisis, the Science Police outfit was a gray bodysuit, matching gloves and boots, and a dark blue stripe down the middle of it - basic, not uni-color, and somewhat interesting. Almost every Legionnaire had that same outfit. Differently colored, with varying accouterments (e.g., emblems, metallic discs, etc.) But it was easy to see that A) they were part of the Science Police (deputized, as I recall), and B) they were a team, shown by design of those costumes.

I guess I'm just a Legion fanatic (of COURSE I am!)

John B. --  SWORD OF THE ATOM "de-uniqued" him?  At the time (and now too, I guess), I thought it did the opposite.  Instead of him being another shrinker (like Doll Man and Ant-Man--hmm...all in blue and red even) who couldn't carry his own series for very long, his power led him to an interesting new environment and raison d'etre.  But, most of all, Gil Kane's new design rocked!  The Atom has not looked better, before or since.  The only thing that was not unique about the situation was that the Hulk also had gone the barbarian route previously in Jarella's world (and also since in "Planet Hulk").  Unless you're referring to actual barbarians like Conan, et al.  But the Atom was now not an ACTUAL barbarian, he was a TINY barbarian!  The only one!  And, hence, unique.