I think a lot of companies still focus on Netscape as their second supported browser out of habit. They also probably aren't doing the research to determine where they really should be focusing their attention.
I work for a web content company and we have recently taken a good look at what our customers are actually using and adjusted our choices accordingly. IE 6.x under Windows certainly still constitutes the vast majority of the customers. As Andrew noted, it is generally around 90% or so. Obviously, it remains our top priority. At the same time, if you stop looking at percentages and instead look at numbers of actual users, it quickly becomes evident that other browsers are worth looking at.
The most recent information we have seen is that Firefox remains at #2. We have also been pretty surprised to discover that Opera is now #3 and gaining. It definitely is getting more attention now that they no longer charge for the ad-free version. While we haven't yet moved to adding it to our "officially supported" list (essentially which browsers we test everything on), it is pretty likely to make the list soon. We do still test with Netscape 7 and up, but it has been losing market share (we were surprised that it is now behind Opera) and could drop off our list.
On the Mac side, we focus almost entirely on Firefox and Safari. The Mac version of IE was on our supported list until very recently, but Microsoft has now discontinued support for the product and its usage continues to drop dramatically. Mac IE is actually very difficult to code to as it is one of the least standards-compliant browsers out there (much worse than IE Windows).
One thing to note is that the latest versions of Firefox, Netscape, and Safari all pretty much use the same underlying rendering engine (known as the "Gecko" engine), which means that if a site works ok on one of them it is almost certain to work ok on all of them. Opera has gotten better, but it still is a bit tricky at times getting pages to work well on it.
IE Windows is not as compliant to the "official" web standards as established by the w3 organization as some of the other browsers, but it so completely dominates the market that it really is the actual standard. If it ever comes down to a decision where we have to make a choice between getting something working on IE Windows and getting it to work on any or all of the other browsers, IE Windows wins out every time. I think part of where Opera tends to get into trouble is that its developers have tended to adhere exclusively to the w3 standards and not tried to match up with some of the quirks of IE. Firefox and the other Gecko browsers have been better about matching some of IE's non-standard features.