I finished the main quest and I am dissapointed that I can't further explore the wasteland. Are there any mods that would let me do this and see how the world has changed? I also didn't activate the machine myself so I survived and I poisoned the water with virus.

Would be interesting to see how the world turns out.

Thanks

Broken Steel allows you to keep playing after the end of the main quest.

If you don't want to buy Broken Steel, http://www.fallout3nexus.com/downloads/file.php?id=586 will allow you to continue to play.

For that you will need something like Broken Steel. Alternativly you can install the "Summer" or alternatives after the MQ is finished in combination with free play after MQ. It should make the wasteland a lot greener in terms of flora.