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LogCloud provides a shortcode that generates a tag cloud with logarithm scaled font sizes.

LogCloud displays a tag cloud much like the built-in WordPress tag cloud, except that instead of scaling the font sizes linearly, the sizes are scaled using a logarithm scale.

If you use your tags with approximately equal frequency, the normal linear scale works just fine. However, if you tag your posts such that just a few tags predominate, the linear scale results in one or two very large looking tags in your cloud, and everything else will use a couple of font sizes at the lower end of the scale. A log scale cloud will result in a more even distribution for this situation.