Waterloo Plugins

Tag Cloud Widget adds a simple tag cloud widget that you can use anywhere on your blog. This plugin looks at all your tags and displays them in a widget. The more times a tag is used, the larger the tag is.
Each tag is a link to the tag page (e.g. http://your-site.com/tag/your-tag). This will help with SEO.
Currently, there aren’t any options to customize the tag cloud. It should work well out of the box. In the future, there may be options to customize the widget.

Good

A tag cloud widget with links to your tag pages

keith_wp

The normal WP Tag Cloud widget only uses one taxonomy at a time, and doesn’t handle the default post_tag or category taxonomies being assigned to custom post types.
With this plugin, you can now configure (on a per-widget basis):
<ul>
<li>which taxonomies and post types are to be included,</li>
<li>the size of the smallest and largest tags (in different font units),</li>
<li>which terms should always be excluded from the cloud,</li>
<li>the maximum number of tags to show in the cloud,</li>
<li>how to order the tags – alphabetically or by number of tagged posts (ascending or descending), or just randomly,</li>
<li>how to treat tags with just one entry – leave alone, remove them or link directly to that post,</li>
<li>whether to make links no-follow,</li>
<li>what case the tags should appear as,</li>
<li>the widget background and tag background/foreground colours,</li>
<li>and how long each widget’s output should be saved for, if at all.</li>
</ul>

Combined Taxonomies Tag Cloud

A widget to make tag clouds out of multiple taxonomies, across multiple post types and control their appearance.

Tag Cloud Widget vs Combined Taxonomies Tag Cloud