ston3d/shiva of course, try their in-browser 3d games on their website!
the player is cross-platform, it runs on windows, mac and (planned) linux. mobile phone/ps3/xbox port in development.
the (dedicated) server runs on many platforms and allows quite some players online at the same time, if you have the cash to buy the full version.
@pappenheimer: the sothink swf quicker is a great tool for flash games, however, i'd not recommend it as a web game solution, because you'd have to code a lot by your own in php/java/whatever.