iMac cleanup: can I delete these files?
Looking just at the big parts


You shouldn't touch anything in /private, as these are files used by OSX.
Removing unneeded drivers from /Library/Printers shouldn't pose a problem (I only have the Canon drivers for the model I'm actually using in there)
.../iTunes/iTunes Media/Mobile Applications contains all the apps you've ever purchased. You can, if you want to, delete those which you are not using anymore (and reload them again if necessary), but anything installed in your phone will most probably get resynced on the next sync
In .../Library/iTunes there might be some old iPhone updates left in iPhone Software Update which can be removed without problems
.../Library/Developer/Shared contains the Xcode and Development documentation, which can be managed via Xcode directly (or you just remove it and use the web-based version)


I wouldn't bother with the smaller stuff (including iDVD and iPhoto).