I love Amazon's wish list, but I have a few items I want to record that aren't available on Amazon.
Is there another app that does something similar and/or a way to add items that aren't listed on Amazon to my Amazon wish list?
You can actually add things to your Amazon wishlists that are not available on the site using a bookmarklet by using use their universal wish-list service
Based on your description "I love Amazon's wishlist" I would say this would be a good option for you
Wishlistr can do this and supports importing public wishlists from Amazon and Delicious. The downside is that it doesn't support letting people tick items off, so you'll have to remove items manually.
Google spreadsheets is pretty nice. You can create a form for easy input and you can create a link for others to view. A mobile site is also available.
There a social wishlist app at http://wishd.me - you can create and share wishlists with your friends.