Mariapan

This plugin allows you to add a tag/category cloud with count widget to your sidebar or use a shortcode to show a category/tag cloud with count in post & page. You can select color combination of cloud, the minimum number of posts in a category to show and which categories to include or exclude.
<h4>Features</h4>
<ul>
<li>Use as a widget or shortcode</li>
<li>Configurable color combination of your cloud display</li>
<li>Order by number of posts in each category or alphabetically</li>
<li>Specify the minimum number of posts that a category has to have before it shows</li>
<li>Specify categories to include or exclude, or use them all</li>
</ul>
<h4>Usage</h4>
Shortcode 
Optionally enter [tagmagic] in a page or post to show the category cloud. See the FAQ for examples.
Title 
This is the usual widget title that will appear in your theme’s sidebar.
Order by
Choose between ordering by number of posts in a category, or alphabetically by category name.
Show by 
Either the category with the most posts first or the category with the fewest posts first if using Order by: count, or A-Z or Z-A if Order by: name.
Minimum number of posts 
Categories where the total number of posts is less than this number will not be shown. Set to 1 to hide empty categories.
Comma separated category IDs 
If you only want to include specific categories, enter their IDs in a list. If you want to exclude a category, enter its ID as a negative number. Leave blank for all categories.
<ul>
<li>
Example: 1,10,20 
This will create a category cloud with only categories 1,10,20 in it.
</li>
<li>
Example: -1,-3 
This will create a category cloud hiding categories 1 and 3.
</li>
</ul>
Parent category ID 
If you only want to include categories cloud of particular parent category, enter their IDs in a list. Leave blank for all categories.
<ul>
<li>Example: 1 
This will create a category cloud of parent category 1.</li>
</ul>

Wp Tag Magic Widget

Display selected tags or categories as a tag cloud using a sidebar widget or shortcode.

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

Wp Tag Magic Widget vs Tag Cloud Widget