Shorter is better! Keep your website address short. Preferably without a WWW a the start, this makes the site address longer and I've seen people screw up the WWW (I've incorrectly seen WW and WWWW before). Always list the site without the WWW (someone geeky can help get your site to redirect to the shorter one).
If your website is http://mystuff.com/ use that address always but for some reason if you need to link to something else such as a calendar then make sure the address is http://mystuff.com/calendar/ not http://mystuff.com/calendar.html. There are several reasons for this.
Never list a address ending with index.html (or .htm, .php, etc) or Default.html (or .htm, .php, etc) for all of the same reasons as above.