.NET version detector is a pretty handy tool in detecting the .NET version installed on your system and download the latest .NET version available. The current version of this tool can detect up to framework 4.0. Interestingly the tool also provides link to download SDK, framework and help you to navigate to the installation folder of each framework on your system.
Here is snapshot of the tool when I used it on my machine.
You can download this free tool from http://www.asoft.be/prod_netver.html.
Hope this helps!