Paint

Microsoft Paint, often called "MSPaint" as a reference to its executable's filename, is an application supplied with Microsoft Windows installations. Considered to be the most simple application for artwork, it is perfectly suited for first time sprite creation and simple sprite sheet manipulation. Due to its ease of use and simplistic nature, there is a very common misconception among artists around the web that Paint is somehow inferior to other graphics editors for spritework. That is simply untrue. Though Paint allows for an easy entry into the worlds of graphics editing and pixel art and therefore commonly being used to create low-quality work by inexperienced users, it is by no means inferior to other programs from a creation standpoint and is fully capable of making pixel art as intricate and complex as even the most powerful of graphics editors.

Versions

While later versions of Windows supply more complex and feature-rich editions of Paint, it's advised to use to the version released with Windows XP due to its small size, ease of use, portability, and ability to work natively across all Windows OSes released after XP as well as Linux (under Wine).
_{(I haven't tested whether or not XP Paint will work with Windows 2000 or 9x releases [I doubt it would run on 9x due to XP being NT architecture but I'll test this out and post an update.] ~Vipershark)}

There is a website known as "MSPaintXP" that offers a download link of a copy of XP Paint, but the site has locked the copy of Paint in an installer which not only downloads some sort of OTHER installer while it's working, but it pops up several ads in quick succession that are designed to trick the user into installing software other than Paint that the user does not want. The XP version of Paint does NOT and has never needed to be installed on any version of Windows and works with full functionality simply by opening the standalone executable. Because of this shady behavior, I don't recommend or endorse using the MSPaintXP site to get a copy of XP Paint. Instead, if you have access to a computer with Windows XP, mspaint.exe can be simply copied onto a flash drive or other means of file transfer to allow you to move it to another computer.

A comparison between the two copies of Paint

If you do not have access to a computer with Windows XP, Vipershark has uploaded his personal copy of MSPaint obtained from a freshly installed copy of Windows XP Service Pack 3, which is available for download below:
mspaint.exe on MEGA (md5 hash: 949bc05cef66bcd68eb23f08eb4c2dff)

The version of Paint that is included in the MSPaintXP installer is actually an older version of Paint and the md5 hash does not match my copy despite their identical file size.

XP Paint

XP Paint, often referred to as "XPaint" when discussing differences between MSPaint releases, is the version of Paint that was included with Windows XP. It is the most commonly used version of Paint and includes a simple toolbar with 16 tools and a palette of 28 colors. It can undo up to three previous actions and is able to zoom in 8x (or 10x if you know the trick to do so.)

Despite its simple nature, it is quite powerful in terms of creating and manipulating spritework and is recommended for both beginners and advanced users alike.

Vista Paint

Vista Paint, often referred to as "VPaint" when discussing differences between MSPaint releases, is the version of Paint that was included with Windows Vista. Due to the controversy and criticism surrounding Windows Vista's release and many users waiting until Windows 7 to upgrade from XP, VPaint is possibly the least commonly used modern version of Paint.

VPaint is essentially an updated version of XPaint which includes the same 16 tools (with new icons) and a palette of 28 new default colors. In addition, it supports up to ten undos as opposed to XPaint's three, and the zoom in/out function has been updated to a new zoom slider which allows for 10x zoom by default.

While VPaint might sound better in terms of its updated features, actually getting it to work on a computer that runs Windows 7 or 8 instead of Vista requires administrator privileges and using the command prompt to replace the Windows 7 version of Paint with the Vista version. While this change is easy to do and only takes a few minutes if you know how, not everyone has access to administrator privileges on their computers and the addition of 7 undos isn't really worth the hassle when XPaint can be used by just opening the program.
_{I'm unsure whether it's possible to use VPaint on XP but I'll test it out and update with my findings although it's highly unlikely that it will work. ~Vipershark}

7 Paint

7 Paint, often referred to as "7Paint" when discussing differences between MSPaint releases, is the version of Paint that was included with Windows 7 and is currently included with Windows 8. Due to the controversy and criticism surrounding Windows Vista's release and many users waiting until Windows 7 to upgrade from XP, 7Paint is possibly the second most commonly used version of Paint.

7Paint has been completely revamped from the ground up and the interface has been completely redesigned. It uses Microsoft's Ribbon interface for the toolbars and supports brushes and anti-aliasing by default. Though it is much more feature-rich than older versions of Paint, it is a lot more cumbersome to use and not recommended compared to XPaint.
_{I'm unsure whether it's possible to use 7Paint on Vista or XP but I'll test it out and update with my findings although it's highly unlikely that it will work on XP and may or may not work on Vista. ~Vipershark}