What about Java. Can you delete all your files, completely wipe your hard drive using a Java applet (hosted on a web site)? The answer is yes, if you accept running the applet. Unlike any other web browser language.
Java has the ability to do things like execute programs on your computer (executables) and also has the ability to write, update or delete files on your hard drive.
Also, Java applets are not detectable by virus scanners: In most cases, you won't even know it screwed up your computer. Some scanners may detect that something is trying to delete files in a restricted directory: One I know is Kaspersky, but most people have this feature turned off by default.
Java applets can do things such as update Windows files, like
HAL.dll, which will prevent your computer from booting. It can do anything to your computer when you accept to run the applet.
In some cases it doesn't even matter if a Java applet is signed or not signed - it will still download files on your computer.
Not to mention Java is very popular.
There's another one that's growing in popularity, called Unity Engine (similar to Flash): It has the same security problems like has Java and could do anything to your computer. The only difference between Unity Engine and Java is that Unity runs without asking you if you wish to run it or not. So if someone has Unity Player installed and runs a game that contains a virus, it will screw up your computer.
Less popular, but could potentially extremely harmful is VBScript. I believe Microsoft Internet Explorer is the only one that supports this currently, but I could be wrong. It has the same abilities as Java.