Transformations -  This is basically a case where it's not exactly the costume but more of the person inside the costume making a change...Peter Parker has been through many different physical changes in his career as Spider-man and here are the most prominent ones:

Six Arm Spidey

Spider-Man grew four extra arms at the end of the infamous Amazing Spider-Man #100 where Peter tried to create a potion that would make him normal once again...After taking it, Peter had some hallucinations then was surprised to learn that he grew the four extra limbs...He went on to battle Morbius in Amazing Spider-Man #101 with the Lizard joining in on the fun in Amazing Spider-Man #102...The extra arms were all gone by the end of the trilogy.

Spider-Hulk

In the midst of battling the Hulk in Web Of Spider-Man #69, Spider-Man was zapped with a Biokinetic Energy Absorber that was in the same vicinity as the Incredible Hulk...Of course this meant huge quantities of Gamma Radiation was present and Spidey went on to Smash!...At least temporarily in Web Of Spider-Man #70!

Spider-Carnage

After battling Carnage in Amazing Spider-Man #410, Ben Reilly was shocked to learn that Carnage vanished in the middle of a huge rumble...He found him later in that issue and yet another battle ensued that ended with a much larger shock to Mr. Reilly: The Symbiote that bonded with Cletus Kasady took a liking to Reilly and thus Spider-Carnage was born!...Ben fought to regain control of himself in Spider-Man #67  and it wasn't until Spectacular Spider-Man #233 that he broke free of it's control and returned once again to Kasady.

Spider-Lizard

At the end of  Spectacular Spider-Man #39 Spider-Man turned into a Lizard like creature resembling one of his most feared foes, the Lizard...In Spectacular Spider-Man #40, it was learned that while Spider-Man was curing Curt Connors of the energy charge that turned him into the Lizard, Spider-Man absorbed radio-active feedback  that affected his blood...Spider-Man was cured after Connors administered a vial of serum into his bloodstream.

Spider-Phoenix

Excalibur guest starred in Spider-Man #25 and they combined forces to combat Arcade...After administering C.P.R. to Phoenix, Spider-Man somehow gained similar powers to her and became the Spider-Phoenix...It was quickly learned that a Bio-Organic Circuitry Board  was webbed to Spider-Man's back and that Arcade was behind the shenanigans experienced by all.

Cosmic Spider-Man (Captain Universe)

Spider-Man first gained cosmic powers in Spectacular Spider-Man #158 courtesy of Captain Universe...The powers were only temporary but it enabled Spider-Man to make quick work of such heavyweights as Magneto, The Hulk, Tri-Sentinel, Goliath, and others.