A SIM toolkit is a set of applications (well..services) that the service provider provides. The SIM can be programmed by the provider to contain such services that their customers can access. These are usually delivered via SMS and you are charged a fee for using them.
SIM applications are widely used everywhere EXCEPT north america :)
These services vary.
Most basic services that most providers seem to have: horoscope, weather, news info, sports
Some more specialized: Secure banking, payments, chat, dating, ship & airplane schedules
Siemens SX66 Blue Angel
ROM Helmi 1.3beta | Radio 1.15.00 | Protocol ver 1337.42
Carriers: Cingular USA + Cosmote GR