Plastic Post Style

To enhance the WordPress default formatter’s flexibility, this plugin
enables configuring content and excerpt styles with various formats.

Features

• Replaces “br” tags with “p” tags.
• Produces valid XML parser for all sources.
• Automatically generates excerpts in chosen length.
  Enables configuring following settings; length, separator elements (period,
  touten, paragraph), read more text, tags which you allow in excerpts,
  and etc..
• Add any tags or texts to one’s contents as header and footer.

Demo

See this plugin working on my blog. Contents and excerpts in this site are generated by this plugin.