The browser version of Pages, Keynote and numbers was announced at the WWDC keynote address and is available to registered developers only.
iWork for iCloud is expected to be released sometime this year and will work on Mac and PC. Chrome, Safari and Internet Explorer will all be supported.
Developers can access the new iWork at the link below. It will also be interesting to see when Apple will update the web based iCloud interface since many of the icons used are form iOS 7.