A really useful metric for internet marketers to watch is "organic traffic %."

Most web traffic comes from:

Service Emails
Marketing Emails
Search
Other Advertising (banners, TV, radio, print, etc.)
RSS Feeds
Organic (people that come without being prompted; a proactive visit, directly to your site)

The breakdown of these sources varies by industry of course, but it's useful to watch how the percentage of traffic that comes organically changes over time. An increase in organic traffic over time is a good way to measure the increased value and quality of your product.