I can no longer use Stack Overflow from my company (I work for a major bank) as IE6 is the standard browser and there are no plans to upgrade.
Why have the designers chosen to discontinue IE6 support?
To expand on @Kop's answer: Internet Explorer 6 was released on August 27, 2001 according to Wikipedia. It is now almost a decade later - how many 9-year-old browsers are the website developers supposed to support? Netscape 4.6? Opera 6? Firefox 0.9? (does anyone still remember those, much less use them? Check this timeline to see what were the state-of-the-art browsers then.)
IE6 is old and very broken. It was created and rolled out to a very different Internet than we see now - it was expected that people will write sites for IE6, as it was a pretty modern browser then, with all kinds of cool features. Since that time, both the Internet and Internet Explorer have moved on - complaining that IE6 has problems on modern sites is like grumbling that the Ford Model T is not well suited for today's superhighways.
Although a lot of sites today still work in IE6 (but see this anecdotal article), what you probably don't see is this: a large chunk of resources goes into the hacks required to keep the sites usable in IE6; resources that could get a better use somewhere else.
The longer we delude ourselves that it's a normal, modern, usable browser, the longer we'll need to bend over backwards to accomodate for it - and the longer the users will say "but everyone else supports this, why can't you?". That last exclamation is not even true anymore - amongst others, even the giants have resigned from creating specific hacks for IE6 - even its maker, Microsoft; why should anyone else?
It's unfortunate that some institutions are clinging to this very old browser; but as long as the web still kind of works in IE6, we'll still see IE6 ("why should we upgrade it? It works for us, we don't care what you had to do to achieve that").
Incidentally, I've just fired up a VM with IE6 (which I keep for the clients who insist that IE6 is the One True Way and won't have it any other way), and SO seems fairly usable from there. It looks a bit ugly, there are obvious rendering bugs, but I don't see anything that would rate as "cannot use SO in IE6". What problems exactly do you see? (Note that these may be the artifacts of your specific IE configuration - certain plugins etc can mess up IE6 pretty badly)
Update: It seems that Microsoft is now trying to get some good PR by jumping on the IE6 Hate Bandwagon, and is actively pushing for its eradication. If that is not a convincing enough reason that IE6 is truly obsolete, then nothing will ever be.
Are you able to ask and answer questions? Then there is nothing keeping you from using ie6 on stackoverflow.
If you find that you cannot ask and answer questions (not including the markdown editor- that might not work) then post a specific bug report here, and if others agree that it prevents users from participating in the most basic site functionality, they may consider fixing it.
Up until a year ago I worked at a company that only used and supported ie6 and I found the site to be perfectly usable. Perhaps they've change the site significantly since then, and broken that support, but I honestly don't remember having problems asking, answering, voting, commenting, etc.
Regardless, a general rant like this is pointless. Post specific bug reports that are reproducible.
10 years ago a browser was born.
Its name was Internet Explorer 6. Now that we’re in 2011, in an era of modern web standards, it’s time to say goodbye.
This website is dedicated to watching Internet Explorer 6 usage drop to less than 1% worldwide, so more websites can choose to drop support for Internet Explorer 6, saving hours of work for web developers
I hope you have a system at home that lets you access http://careers.stackoverflow.com/ as it seems you need it!