txt2html

txt2html is a program to convert plain text files to the hypertext markup language.
Extremely clean W3C 4.01 Strict DTD compliant HTML

• Option to include your own CSS
• HTML entity replacement (optional; you can also edit the entity list). Note
  that critical entities (<,>,",&) are always replaced, so generally you
  don't have to enable this option!
• Detects _italic_ and *bold* (optional)
• Detects and marks URL's (optional)
• Optional paragraph joining
• Optional shell integration (item "txt2html" in right-click menu for .txt
  files)
• Fast
• Free