LogCloud provides a shortcode that generates a tag cloud with logarithm scaled font sizes.
Unpack logcloud to your plugins directory (preferably using handy-dandy built-in install tools), then activate it.
To use logcloud, insert the [logcloud] shortcode anywhere you want a tag cloud to be.
The following attributes may be added to [logcloud]:
LogCloud also adds a widget that can be used in your sidebar if you have widgets enabled. Options are the same as above, except the link option is not available.