I would like to monitor how much data-traffic I use. How?
This is now built into Android in Ice Cream Sandwich. You can view usage per application, set limits and restrict background data.
Some more information: http://androidadvices.com/set-mobile-data-limit-usage-ice-cream-sandwich-devices/
NetCounter monitors mobile and wifi traffic.
Another option: My Data Manager
When installed it keeps track of your plan type (monthly or pay-as-you-go), amount of data included per month, and the 1st day of your billing cycle. You may also set your plan's roaming details. From then on it will log your use of mobile data, roaming mobile data, and Wi-Fi data used. You may setup notifications for each of the three categories when they hit a certain amount of data.