The final product in the QuickBASIC line was QuickBASIC Extended 7.1, also called Microsoft BASIC Professional Development System 7.1 or QBX. (According to Wikipedia's QuickBASIC entry, the QBX IDE did not run under OS/2.)
QuickBASIC was succeeded in 1991 by Visual Basic. Despite being sold as part of the VB line of products, Visual Basic for DOS (VBDOS) is actually a QuickBASIC compiler and can compile most (if not all) QB code unchanged. (It might help to think of VBDOS as "QuickBASIC plus character windows".)
It should be noted that while QuickBASIC is not 100% compatible with BASICA and GW-BASIC (the interpreters included with older versions of DOS), most BASICA/GW-BASIC commands work identically. (If a program is marked as working with GW-BASIC, there is a strong chance that it will work in QuickBASIC. The QuickBASIC help file includes a side-by-side comparison of incompatible features of the languages.)
A version of QuickBASIC was released for the Macintosh, but it doesn't run on anything newer than System 7 (and even there has some limitations) and is therefore limited to m68k machines. (For non-Mac users, this is somewhat similar to a Windows program being able to run under Windows 3.x and Win9x, but not WinNT.)
The Mac version was canceled after version 1.00, likely due to lack of sales.
QBasic is an interpreter that Microsoft distributed with later versions of MS-DOS, replacing the classic-style interpreter (GW-BASIC) that was included with earlier versions. It is based on QuickBASIC 4.5, with the compiler and parts of the language removed. Almost any program specifically written for QBasic will work under QuickBASIC 4.5 or later without changes.
(Note that it is not correct to refer to QuickBASIC as QBasic -- Microsoft considers them to be two separate products.)
The QuickBASIC language has become something of a de facto standard, inspiring several later compilers, many of which attempt to be QB-compatible (with varying degrees of success). A few of the modern compilers include: