The setspace.sty package allows for double (and 1.5, etc) spacing. Here is the basic documentation.

Usage: in the preamble, put \usepackage{setspace}

You may also put in the preamble one of the commands: \doublespacing, onehalfspacing, and \singlespacing. These set the overall spacing for the document. Single spacing is the default.

You can get other spacings by using the \setstretch{baselinestretch} command in the preamble to set the baselinestretch appropriately. The default spacing with this style option is single spacing.

There are three environments that can be used to set parts of the document in different spacings: singlespace, onehalfspace, and doublespace. (Both the onehalfspace and doublespace environments are intended to increase the spacing, so the onehalfspace environment should not be used in a double spaced document). If an increased spacing different from one-and-a-half or double spacing is required then the spacing environment can be used. The spacing environment takes one argument which is the larger baselinestretch to use, e.g., \begin{spacing}{2.5}.

Since you can't see the effect of spacing in html, you should look at the dvi, ps, or pdf versions of this document to see what the same piece of text (including a footnote and a linguistic example) looks like set at different spaces.