Unlock alternative costumes:
As you may know, to unlock the alternative costumes you have to follow a different path than on xbox 360 and ps3 consoles. We don't get to see spider emblems, take photo of them and all the costumes just in minutes.. Instead we get the hard way. Here's is a list I've got so far by playing about how you can unlock some of the costumes on PC VERSION of the game.
1) CLASSIC BLACK SUIT(Sam Raimi Movie Style)
Just get progress through the story and shortly you'll recieve it.
2) SCARLET SPIDER(2012)
Collect all the 700 comic books around the city. After you hit 500, you can see the location of the remaining comic books in your minimap, but not visible on the big map on your phone. Just a hint, I searched around looking for only a single comic book which was the only thing left to complete and I found it when I got up to the biggest building in the city, that huge black oscorp building where you fight the snake robot. Until you get up there, it's not shown in the minimap because it's so high.
3) CROSS-SPECIES SPIDERMAN
Finish the game on any difficulty.
4) FUTURE FOUNDATION SUIT
Finish all the green side-missions.
5) NEGATIVE ZONE SUIT
Finish all the blue side-missions. (Like petty crime, car chase etc. Don't get worried when you don't see any more blue missions without getting %100 percentage on these side missions. They're likely to appear later at a time for you to progress further)(supplied by: Mudeku)