Hide the Left Navigation

In SharePoint 2007

1. Go to the page where you wan to hide the left nav. 
2. Add a content editor web part. Go to source editor.
3. Add this content. <style>.ms-navframe{ display:none; }</style> and save.

In SharePoint 2010, you need to add the following to Content Editor Web Part
<Style>
body #s4-leftpanel
{
display: none;
}
.s4-ca
{
margin-left: 0px;
}
</style>

In SharePoint 2013, you can do the following to hide Quick Launch:

<style>
.ms-core-navigation { DISPLAY: none }
#contentBox { margin-left: 0px }
</style>

Please note that if you want to do this for all pages within a sub-site – then it would be more preferred to add  above code in the applied master page.