When I set up a site in multiple languages, how should I set up my urls for search engines and usability?

Let's say my site is www.example.com, and I'm translating into French and Spanish. What is best for usability and SEO?

Directory option:

http://www.example.com/sample.html
http://www.example.com/fr/sample.html
http://www.example.com/es/sample.html

Subdomain option:

http://www.example.com/sample.html
http://fr.example.com/sample.html
http://es.example.com/sample.html

Filename option:

http://www.example.com/sample.html
http://www.example.com/sample.fr.html
http://www.example.com/sample.es.html

Accept-Language header:

Or should I simply parse the Accept-Language header and generate content server-side to suit that header?

Is there another way to do this? If the different language versions don't have different urls, what do I do about the search engines?