The first microcomputer in an appreciable quantity and with competitors
1975 was the time of microcomputer pioneers. MITS offered a 'kit' with an Intel 8080 based CPU card and 256 byte memory*) in a basic version, terminal and other peripherals not included. Later floppy disk subsystems and several other extensions like I/O cards and memory extensions were also sold.
A picture shown in Popular Electronics Januar 1975 (this was the first one):
Ad shown in the first issue of BYTE, Septemper 1975:
Very forward-looking was the design of the I/O system, a backplane with a system signal bus system - later called the S100 bus (a lot of documentation can be found on Hartetec's Manual pages).
The first commercial selled software was the Altair BASIC 4K and 8K interpreter (in fact from Bill Gates, Paul Allen and Monte Davidoff developed, they earned $30 for every selled BASIC) - on paper tape.
A disk operating system named "DOS" (not related with MS-DOS of course, also mentioned often as Altair DOS) was the basic operating system for disk drive operation (8" drives from Pertec), a bit later a CP/M compatible OS, "IMDOS", was implemented on (and for) an IMSAI 8800.
No computer enthusiast nowadays can imagine how difficult it was to start a program on an Altair 8800. You have to enter the bootstrap code with the front panel first (bit-wise of course).
Today it's a rare computer (8800A). When it was selled, it was ment for a customer who has to know how to build a computer, how a micro computer works in detail and how machine code should be used - for users from the year 2008 very strange.
Because the Altair's were only selled from specialist dealer, with the IMSAI 8800 (an improved Altair 8800 clone in fact) a strong competitor - not only selled by a few ones - was early on the market, too:
The last "official" Altair was build in 1977, after Pertec buyed MITS, but Pertec closed their microcomputer branch in the same year.
Please visit the very good and detailed S100 related pages from Herb Johnson, too (also included is also a very good detailed info about the Altair itself there).
*) implemented in pairs of 256x4 RAM chips.