Personally, I don't think there is much we have Bill Gates, or Microsoft in general, to be grateful for. Altair BASIC
WP maybe, but from there on it went downhill

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Upfront I offer my disclaimer: I'm a long-time UNIX fan, and I use Microsoft products only when really really needed.
He forced big companies like IBM and Intel to get into the home computer market. As innovative and creative as guys like Steve Jobs were, they just didn't have Gates' drive and savvy.
There's nothing really to credit Gates with the invention of the PC. IBM had already chosen to go into the small computer market, had picked Intel over Motorola, and almost as an afterthought decided they'd have to deliver an OS with it too. The first version of MS-DOS (86-DOS
WP) was not more than a rewrite of CP/M for the 16-bit 8086 processor. Gates can only be credited with the business savvy that he sold IBM an OS he hadn't even yet purchased from his competitor SCP.
At the time that IBM brought the first PC to the market - with only a text interface - Apple was already working on the Lisa and the Macintosh, the first computers with a graphical interface. It would cost Microsoft nearly ten years to come up with a halfway decent GUI (Windows 3).
The real driver behind the popularity of the PC has of course been the price. That's due to IBM; not for their own prices, but for their decision to design the PC in a modular fashion, which made it possible that other manufacturers produced clones that adhered to the same specs. Compaq is to be credited more with getting the PC in every household than Microsoft is.
Personally, I view it as an over all positive. Sure, in recent years, MS products have been lacking (some were never that great to begin with) and many of his business practices seem to stifle innovation (irony), but the fact that we can get on an internet message board to curse him for these things says a lot too.
The internet is a salient example how Microsoft has not contributed positively to technology. To begin with, Microsoft massively missed the bandwagon of the internet rise in the mid 90s. Windows 95 came without a decent TCP/IP networking stack (see Winsock
WP) and without a browser. The early versions of Internet Explorer were vastly inferior to Netscape and tried to subvert Netscape's advantage by, a.o., posing to webservers as if they were Netscape. Microsoft has perverted every internet standard in sight - HTML, Javascript, CSS - so that to this day web developers who know the standards have to spend extraordinary time to make their pages look and work good in IE as well as in all other browsers. Microsoft has done in this way more to set the internet back (or not having it progress as fast as it could have) than to move it forward.
When you look carefully, Microsoft has never been a technology leader. IE, Word, Excel, even MS-DOS itself were all products that had been developed at other companies and bought by Microsoft. They have been pushed on the public by clever/aggressive marketing and by introducing artificial barriers for the competition. Windows-3 had a check built-in that it would only run on top of MS-DOS, not on competitor DR-DOS. Netscape was pushed out of the market by providing IE for free, and the same is happening with Real and Windows Media Player. .NET is pushed the same way and is fundamentally not different from the Java platform.
He also had the gumption to chase down what he wanted and make the most of opportunities presented.
Yes, Bill Gates/Microsoft certainly took the opportunities presented, but never as a visionary who developed new technology; they always lagged behind and then had to play catch-up with foul means. The headstart given by the contract with IBM over MS-DOS, and the subsequent stranglehold over the PC platform, is basically what has provided them with enough leeway to have that attitude.
Even while I curse out the licensing issues with MS, prepare to put Ubuntu on my computer at home because Vista is driving me insane, and am encouraging my office to switch to more open source applications, I still know that myself and the IT field would not be as well off today without his work.
The IT field definitely looks different today due to Bill Gates' work, but not for the better.
It consists of web developers tearing their hairs out for having to cater their websites to a (dominant) browser which doesn't adhere to the standards; while other web developers (who don't deserve that name) make sites that only are tested in IE and look horrible or don't work in other browsers.
It consists of loads of ignoramuses who, by following "point and click" recipes, got their MCSE and now think they are a system administrator. They don't have the intelligence to get that repetitive tasks can be automated (and when they have, Windows does not provide a decent scripting language for that). They also are raised with the idea that a server must have a graphical user interface too. They've only been fed what's in the book and they haven't learnt the abstraction to look at their systems from a higher level and come with alternative solutions that might better suit their needs - and often, the Windows (administration) GUIs don't offer the ways for alternative solutions.
It consists of a population-at-large that has learnt that computers are not stable and that it's perfectly normal to regularly press Ctrl-Alt-Del and reboot the thing. I don't know if XP, which was quite stable, has been around long enough to make this instinct die out.
It consists of 90% of the email of today being spam, largely being distributed by hacked Windows boxes of individuals with a broadband connection. As a non-Windows user, I thus have to pay the price for Bill Gates' negligence in providing a safe OS. There are cardinal design errors in the Windows services which make it inherently unsafer than UNIX.
It consists of millions of ordinary users struggling with stability issues due to faulty or conflicting drivers, or due to the security compromises that Microsoft has made for better "interoperability" of its products. I bet that, if you factor in the hours spent by nephews, neighbours or friends on nursing Windows installations, the TCO of a Windows box is in the end higher than that of a MacOS box.