After 4 cruises we never go without:
Cruise docs
Solarcaine/sun tan lotions lots of it!
Clothes pins for the clothes line
Quarters for the washer/dryer
Air freshner for the bathrooms
We don't use an extension cord, we found a plug in with 3 or 4 more outlets on it that works wonderfully without the extra bulkiness of an extension cord.
Pop up hamper, the best thing ever! It really keeps things neat in a small place and fits in the closet. It is mesh with handles so easy transport to the laundry room. About $5 - $6 at
walmart
Ziplock bags We thought people were joking about this, but we are glad we have them every cruise so far!
A roll of duct tape and markers/highliters. Duct tape is king!

Oh The days it has saved!
Extra memory
Chargers for batteries, ipods, computers, and phones.
I use papaya enzyme pills instead of rolaids or tums, it's the natural thing, and this has to make it in the suitcase.
Bonine
A cheap shoe organizer, Ikea had them for .99 we hang it in the closet and it gives us extra shelves where we put laundry detergent, bounce
things we seem to collect as the days go by, odds and ends to pack at the end. It's in the closet and out of the way but very accessible.
We have a plastic bin marked cruise box where all our stuff goes. As we think of something we toss it in the cruise box so we won't forget to pack it. I print a list and tape it inside the lid then check it off as we pack the item. Keeps me sane and hubby happy.
Hope this helps. Have fun! I can't wait for 2010 to get here! Our next cruise!