Collecting the Mutants and Mastermind RPG
Mutants and Masterminds, in terms of being collectible, is still a pretty new game. I was the first person in my gaming group to get turned on to Mutants and Masterminds, but my buddies have been running a campaign of their own now for years. It's not in its third edition, so you can collect all three editions of the game plus supplements for all three editions, too.
About the Mutants and Masterminds Roleplaying Game
Steve Kenson designed Mutants and Masterminds for Green Ronin. It's a superhero game using the d20 system, but it's a loose adaptation of those rules. The game plays like a much-simplified version of Champions with fewer dice.
One of the most innovative aspects of the Mutants and Masterminds system when it came out was that it did away with hit points altogether. Instead of hit points, characters in Mutants and Masterminds have to make saving throws versus damage. Depending on how well they make their saving roll, or on how badly the miss their saving roll, they can suffer effects ranging from penalties on future saving throws to being stunned to being knocked unconscious or killed.
Mutants and Masterminds was published in 2002, and the third edition just became available this year. The new edition has the same rules as their new DC Comics based roleplaying game, DC Adventures.
Mutants and Masterminds Products You Could Collect
The first step to collecting Mutants and Masterminds would be to collect all editions of the main rulebooks. The first edition is actually pretty hard to find, but second and third edition rulesets are easy enough to find.
The next step to filling out your collection of Mutants and Masterminds is to start buying the first edition supplements, then the second edition supplements, and finally, the third edition supplements.
Time of Crisis is one of the earliest Mutants and Masterminds adventures, and it's still available at affordable prices on eBay. Lockdown is another Mutants and Masterminds adventure that's not too hard to find.
Some Mutants and Masterminds products had small print runs and were very popular with fans, so they're harder to find and more expensive when you do find them. For example, the Nocturnals supplement isn't easy to find.
The single most collectible Mutants and Masterminds product ever is the sourcebook called Ultimate Power. Apparently it was hugely popular with the fan base, and the print run was too small to meet the demand. If you can find a copy of Ultimate Power for the game for less than $100, you should scoop it up. (I've seen copies of Ultimate Power listed for sale on Alibris for as much as $200.)
M&M Superlink Products to Collect
Once you've collected all of the official Mutants and Masterminds products published by Green Ronin, you can start collecting the various products published under the M&M Superlink license. This license allowed publishers to create their own supplements that were compatible with M&M. According to the Wikipedia article about Mutants and Masterminds, over a dozen publishers have published over 50 different supplements using this license. (That being said, it's anybody's guess as to how long it's been since the Wikipedia article has been updated on the subject.)
An example of a company that produces Mutants and Masterminds Superlink products is Blackwyrm Games, who produced The Algernon Files. Z-Man Games and Spectrum Games have also published a series of supplements in their "Omlevex" setting. These supplements are compatible with other superhero roleplaying games besides just Mutants and Masterminds, including Champions and Silver Age Sentinels.
Many Superlink products for the game are published in PDF format. If they're available in PDF format, then they're not really what I would consider collectible. Collectors prize items as objects as much as for the information that they contain, and a file on a computer is not an object I can pick up and look at and enjoy.
The latest edition of Mutants and Masterminds coincided with Green Ronin's publication of DC Adventures, which is basically just Mutants and Masterminds for the DC universe. So if you love Mutants and Masterminds and want to play Superman and Batman, DC Adventures is the game for you.