Windows

All Windows server versions, as well as some desktop versions of Windows (2000 Pro, XP Pro, XP Media Center Edition (MCE), Vista Ultimate, Windows 7 Ultimate) have integrated remote desktop servers.

HTML 5 (browser based)

With the advent of HTML5 (specifically WebSockets and the <canvas> element), it became possible to create browser based remote desktop clients that run without requiring a browser plugin. These are installable gateway server programs that deliver an HTML5 experience to any compliant browser without requiring a plugin to be installed.

• ThinVNC – an open source HTML5 remote desktop client

  ThinVNC web site
  The SourceForge project

• RemoteSpark Spark View

  Spark View web site
  Spark View online demo

• Ericom AccessNow

  AccessNow web site
  AccessNow online demo

In case you're interested in the inner workings of HTML5 clients, here's an article on the topic: How HTML5 remote desktop clients work

Linux/Unix

There is also an open source Remote Desktop server:

• xrdp – a Remote Desktop server for Unix/Linux

  xrdp web site