It's not hard. The only problem is that most sensors use 3.3V. But the Arduino has 3.3V onboard for those sensors.
How can i determine, *in general terms*, which sensors i can use without any converters?
The 3.3V pin on the Uno is a 3.3V regulated, positive supply. You can use this to power your Bosch sensor or the ITG3200 safely. You still need to be aware that any digital output from the Uno to your sensor will be 5V and should have either a small transistor or at least a resistor voltage divider to knock that voltage down.
With respect to the Sparkfun ADXL345 board, they run the ATMega at 3V. While it's plugged in to a USB 5V port they never use the 5V power for anything other than the FTDI (USB to serial) chip. The ATMega will see the 5V TX/RX signals from the FTDI chip but the accelerometer never sees any 5V signals from the ATMega.
Ok soldering a resistor of transistor should not be too hard. There is no option of NOT connecting the ouput of the UNO to the sensor? This may sound stupid but i thought that, since i only need to READ values from the sensor it could work? But if i'm not mistaken, since i use I2C, their is only one connection pin between UNO and sensor?
Can the UNO also be run at 3V? That would solve my problem at once i would say?