I found this awesome site that compares market shares globally between browsers. As of 2008-10-01, these are the biggest browsers in the world:
46% – Microsoft Internet Explorer 7
25% – Microsoft Internet Explorer 6
19% – Mozilla Firefox, any version
7% – Safari, any version (most likely including other browsers built on WebKit, such as Camino)
3% – Other browsers (Google Chrome, Opera, Netscape, etc)
Notable is also that Safari versions below 3.0 are at less than 0,02% of total market share.
At least in my job, knowing what browser your customers use to browse the web is crucial. Yet, you hear many different numbers from many sources, and you end up just making up your own numbers that seem somewhat correct. Different sites can be browsed by completely different browser users. For websites aimed at very tech-savvy people (think digg.com) generally Firefox has a much larger market share, whereas a site like myspace.com probably has less Firefox users than even the overall average market share of the browser.
When making a web service, it’s very important to recognize your limitations – but also your possibilites. Modern browsers support features older browsers don’t, and if you know that 75% of your audience uses a modern browser (such as Internet Explorer 7+, Firefox 2+, Safari 2+), it’s sometimes acceptable to design a feature aimed at these people, and doing a light version for the others. For me as a designer, a classic example is PNG-24 images, semi-transparent wonders that makes IE6 totally crap out. Therefore I do a less pretty version of the semi-transparent images for that browser, and most people that actually cares are happy in the end.
It’s an interesting and forever evolving topic, that’s for sure.