ATOI

The main goal we wanted to achieve by writing this plugin was delicate tags appearance tweaking. Now plugin supports tag color and size tweaking, pretty urls, automatic and manual tags and tag cloud output, tag preview and tagged posts count. Such features as tag aliases and search by tag are planned to be implemented in next release.
We are glad to introduce this plugin to the community. If you think that it is necessary and there is need in developing it further, we will do it.
Bug reports and feature requests are welcome.
<h3>Example</h3>
There are 100 posts with tags as your site. 
There are four tags defined: tag1, tag2, tag3, tag4. 
Tag1 has 10 posts, tag2 has 40 posts, tag3 has 20 posts, tag4 has 70 posts (post can have more than one tag). 
There are 3 colors (color1, color2, color3) and 2 sizes defined (10px, 30px). 
So there are 3 color categories and 2 size categories. 
If a tag has less than 33% of tagged posts, it gets into 1st color category. 
If it has more than 33% and less than 66% of tagged posts, in gets into 2nd color category. 
If it has more than 66% of tagged posts, it gets into 3rd color category. 
The same thing with colors. 
So: 
tag1 – 10 posts (10%); color:color1; size:10px; 
tag2 – 40 posts (40%); color:color2; size:10px; 
tag3 – 20 posts (20%); color:color1; size:10px; 
tag4 – 70 posts (70%); color:color3; size:30px;

Xata33 Tag

The main goal we wanted to achieve by writing this plugin was delicate tags appearance tweaking

jfoucher

This plugin uses the yahoo.com API to find the most relevant keywords from the content of your post, and then adds them as tags.
The tagging is fully automatic, so if you’re using a plugin like feedwordpress to display RSS feeds on your blog as posts, everything will get done as the feed posts are published. No user intervention necessary!

Auto-tags

Automatically add relevant tags to new posts.

Xata33 Tag vs Auto-tags