Although the Lumia 900 and other Windows phones of its generation hit something of a dead end in software development, Microsoft's Windows Phone 7.8 update gives them the appearance, at least, of being Windows 8 phones.
OS 7.8 installs resizable widgets that let you cram or expand more-dynamic live tiles onto the home screen at will. You'll also find an expanded color palette at your disposal, and the ability to add a Bing photo of the day as your lock screen image. The software also makes some performance tweaks behind the scenes.
After your update, zip on over to the app store for fresher versions of Nokia-only apps.