iFrame

Simple:

{{url>http://www.example.com/somepage.html}}

Complete Syntax:

{{url>someurl width,height noscroll noborder alignment|alternate-text}}

That is an URL to embed, followed by several optional parameters. The following parameters are recognized:

width and height are optional. When only one value is given it is assumed to be the height. Otherwise the first value is width and the second value is height. You can give the values in px, em, pt or %. When no unit is given px is is NOT assumed (you have to give the value). Defaults values are: width - 98%, height - 400px.
noscroll will disable the scrollbars of the iframe
noborder will disable the frame border, note that your CSS might still add borders or may disable them without using this setting
Optionally an alignment can be given as left or right to have the iframe floating to the left or right. Should be used with a width.
An alternate text is optional. If not specified an empty string will be used.