It's not the site I visit most often (that's LJ), or the one I find most directly useful (Google), but it's my favourite. It's a piece of glorious CSS evangelism, demonstrating what's possible with CSS-based web design. It's effectively a one-page site, with many different stylesheets, all working on exactly the same static HTML document, and achieving wildly different layout effects. The home page remains the same, but there's an often-changing selection of user-submitted stylesheets, of which my current favourite is Museum (remember, that's exactly the same HTML as the homepage I linked above - it's just the stylesheet that's different). I go back to it over and over, looking at favourite designs, getting ideas from the CSS, and being re-enthused about the whole thing, and it gives me so much joy that I want to share it with you ;-)
Now, repay the favour. What's your favourite website, and why?