What ways do hackers employ to gain access to data travelling a network?

I have been learning about ARP Poising using cain and abel, but I am interested in other ways this can be done.

Social engineering, mainly...

Thanks for the reply, I have been getting a list together and wanted to make sure that I aint missed any.

This is what I have so far, have I missed any?

Dictionary attacks
Sniffing
SYN flooding
Smurf attack
Denial of Service
Software bugs
Bad input
Buffer overflows
Social engineering

basically, but they can all be subdivided.

DOS and dDOS. dDOS can also be done non-maliciously e.g the slashdot effect

Get some CEH Prep guides - those books are good for listing lots of tools.

You can also head over to insecure.org and view the list of tools they have there to see if there's anything else you might've missed.