You can virtually test anything and everything online… there is amazing amounts of tools available to measure different things.
Measure traffic? Use Google Analytics (#1 if you don’t have basic traffic measurement installed please bang head against wall – it should be an assumption.)
Want to do Brand Monitoring? Check out Techrigy (Google Alerts on steroids)
Usability and heat maps? Use CrazyEgg (it beats Google Analytics Heatmaps hands down.
Measure E-Commerce? Set up Google Analytics Goals (and figure out where your business comes from).
Measure blog subscribers? Use Feedburner.
Want to find out which design / slogan/ offer creates the most business? Use A/B testing with Google Website Optimiser.
Use these to test your assumptions, tweak and improve again. The web is a huge incubator for evolution or products to happen, so get evolving.
If you have any others feel free to share below in the comments…