If you want to get into packet manipulation, then I would highly recommend Knoppix STD.

Not only does it have your sniffers, it has several programs (the ones perviously mentioned, such as nemesis) and it also has several other ones, including programs for altering data as it comes in (if I remember correctly). As well you can configure for Cable, Lan, DSL, or Dial-up very very simply and everyone can make use of a bootable linux CD. Unfortunately I don't have the link atm. Can't access google for some damned reason. Anyways I'm sure you can find it rather simply.