8) Morph Ball
First Appearance: Metroid (1986)
Perhaps Samus Aran's most iconic powerup is the classic Morph Ball. This item is typically one of (if not the) first items available in the game, with both the original and Zero Mission having immediately to the left of where you start the game (it's located in the same place on Super, but you don't get it for a few minutes). Many games let you start with it already available as an ability.
I am going to cheat a little bit in this post by including all of the upgrades you can get specifically for the Morph Ball. The Boost Ball allows you to charge up your ball form and rocket yourself forward. Not only does this allow you to move faster, but you can utilize various half-pipes located throughout the planets you visit to reach ordinarily inaccessible places. The Spider Ball allows you to traverse specific tracks to once again access places you normally couldn't, but in Metroid II (the item's introduction) you could traverse literally any unobstructed surface, unlocking a plethora of new ways to explore planet SR388. Lastly a minor upgrade comes in the form of the Spring Ball, which allows you to jump while in Morph Ball form (eliminating the need for those pesky bomb jumps).
One other fun bit of trivia: when you're scanning various Space Pirate logs in the first Metroid Prime game, you may come across a log entry that details the Pirate's attempts to reverse engineer some of Samus' abilities, including the Morph Ball. However, they ceased testing after the first few test subjects where left "broken and twisted". Ouch.
8) Speed Booster
It lets you run fast! But in all seriousness it's great because it lets you tear through enemies and environment like it's nothing. I love the idea of also being able to stop and basically fly up or sideways for a few seconds. The speed booster is just all around fun and a great power up.