Dawn of the Personal Computer: From Altair to the IBM PC
Compaq Portable

A few guys from Houston got together to design the first true PC clone—on a placemat, no less!
Sure, at nearly 30 pounds it wasn’t a lightweight, but it was “portable” and, most importantly, you could run Flight Simulator on it.
They reverse engineered the BIOS so as not to be stuck with IBM royalties or legal issues and quickly built one of the more successful computer companies.
In the long run they beat IBM to the punch in releasing a 386 based machine and they stuck with the ISA bus when IBM went to Micro Channel. For these reasons and others they outlasted Big Blue in the PC market.

RAM 128K-640K
CPU 4.77MHz 8088
Supporting OS MS-DOS, CP/M
Predecessor IBM PC
Successor Later Compaq machines
Notable firsts First black-box reverse-engineered IBM PC clone
IBM PCjr
IBM tried the home computer thing, too. Although the IBM PCjr was highly publicized and much anticipated, the reality of the machine was well below expectations.
Sure, it was a powerhouse as compared to the Atari’s and Commodores it targeted, but it also cost hundreds more and it came with a really crappy keyboard and awkward expansion via sidecars.
And, on top of that, it wasn’t 100% IBM PC compatible!
In the end, being a 16 bit player in an 8 bit home computer market wasn’t enough to save this little peanut.
RAM 64K-640K
CPU 4.77MHz 8088
Supporting OS IBM PC DOS
Predecessor IBM PC
Successor None
Notable firsts First IBM home computer
IBM PC XT
A hard drive and an OS with subdirectories don’t seem like much now, but in 1982 these were hawt! IBM and Microsoft weren’t the first to deliver either technology but they were the first to really mainstream it. Before the XT, hard drives were a luxury. After the XT, they were expected.
IBM also killed the cassette port, added slots and beefed up the power supply in recognition of the real users of their technology and in acceptance of their earlier “misunderestimation” of the PC market.
RAM 64K-640K
CPU 4.77MHz 8088
Supporting Operating Systems PC DOS, CP/M, UCSD-P
Predecessor IBM PC
Successor IBM PC/AT
Notable Firsts First IBM PC with a hard drive
Apple Macintosh
Introduced in a now-world-famous Super Bowl ad, the Apple Macintosh wasn’t the first GUI PC — it wasn’t even Apple’s first GUI PC— but it was the little computer that could and it sparked a revolution in personal computer design while redefining people’s expectations of PCs.
When introduced, the Macintosh was underpowered with only 128K of RAM and a slow Motorola 68000 processor, but popular technology and rapid-fire upgrades helped it hold its own.
The Mac not only represented a new technological age for Apple, it defined a break from their “open architecture” standards of the Apple ][ era. With the Mac Apple rebuffed the aftermarket and saw their market share decline dramatically.
RAM 128K
CPU 8MHz Motorola 68000
Supporting OS MacOS
Predecessor Xerox Star, early Apples
Successor All later Macs
Notable Firsts First “affordable” GUI-based PC