Samus Aran's Power Suit has several interchangeable, upgradeable components. One of those is her Visor. While primarily used for seeing things, the Visor can also be altered to perform other useful visual functions.
Most, though not all, visor upgrades appear exclusively in Metroid Prime and Metroid Prime 2: Echoes, since they are the only ones played from a first-person perspective. An X-Ray Visor was also available in Super Metroid, but unfortunately it could not be used to see through her suit.
Every one of these visors also has the capability of locking on to and following hostile targets (this has the additional effect of allowing guided missiles to follow the target, and power beam shots to, er, bend towards the target).
The Combat Visor is Samus's standard visor for blasting stuff away. Aside from the Heads-up display of upgrades, ammunition, and nearby danger, this visor has no special features other than the simple ability to see through it.
The Scan Visor, also standard issue, can recognize objects and scan them into Samus's log book. She can use this in all of the Metroid Prime games. Her suit's internal computer and memory banks then coordinate the scanned image with known data, sometimes inferring new data from the context and environment. Some computer devices can also be remotely manipulated and controlled through the scan visor, e.g. electronic locks. The Scan Visor has the nifty ability of instantaneously freezing Samus in a near-cryogenic state while she reviews information in it, which has the effect of stopping time throughout the rest of the universe for her.
The Thermal Visor, seen only in Metroid Prime, observes and signifies heat differences in the environment. It can be used to sense electrical conduits, and in dark rooms to detect potentially hostile life-forms.
The X-Ray Visor, seen only in Metroid Prime, Metroid Prime 3 and Super Metroid. It uses X-Ray radiation to penetrate and see through walls. Therein, it is useful in detecting "fake walls" that can be blasted away to open secret passages and rooms. It is also useful in following otherwise-invisible Chozo ghosts.
The Dark Visor, seen only in Metroid Prime 2: Echoes, is tuned with the dimensional rift on planet Aether. Some creatures and objects have a habit of phasing in and out of dimensions, making them difficult to track. The Dark Visor is capable of seeing through this tomfoolery and foiling any attempts at invisibility or escape.
The Echo Visor, seen only in Metroid Prime 2: Echoes, is a sonar detection device - sending out sound waves in pulses and filling out the environment based on the response time and angles of returning sound waves. While not very useful for distinguishing gas from holes in the ground, the Echo Visor can see sonically sensitive nodes, used in a handful of puzzles.
The Command Visor is used in Metroid Prime 3: Corruption as a visor that Samus uses to command her Gunship to do various things such as lifting heavy objects and bombarding enemies with missiles. When it is used, it will say Data Recieved while Samus presses buttons on her arm cannon. There then will be a cut scene showing her gunship doing the command that was needed to be done. Although it is only used in Prime 3, Samus can be seen using it at the end of Metroid Prime, when she flees the collapsing Impact Crater.