How can I use iMac internet in my iPod touch?
Yes !

You can share you internet connection via Wi-Fi in the "Sharing" preference pane of System preferences. Once activated, your iPod touch will detect a Wi-Fi network that you can join to share the internet connection of your iMac.

(if you're on Lion, replace "Airport" by "Wi-Fi")


Open System preferences
Click on "Sharing"
Click on "Internet sharing" (but don't check the checkbox yet)
Select "Ethernet" in the dropdown menu "Share your connection from"
Check "Airport" in the list "via"
Click "Airport options" to configure wireless network security
Check the checkbox in front of "Internet sharing" in the list on the left to activate it.
If Airport is turned off, it'll ask you if it can turn it on, say yes.
Connect to the created network with you iPod touch as you would do for any wireless network.