The garment bag is a must have if you have a wedding dress or any kind of formal attire that would like to keep protected. Off season jackets and coats might be a good option for storage too. Moths, fabric yellowing, and wear and tear are some of the dangers that clothes face if not in some kind of covering.
Adjustable wooden drawer dividers are a wonderful way to make you feel like you have built-ins for your drawers. These have unlimited uses for t-shirts, pj's or shorts. The best thing about this product is that you don't have to fold things, and they are still contained & look organized!
Over the door hanging hooks are good for just about anything accessory wise such as hats, scarves, or extra clothes you just don't want to hang up at that moment.
As far as shoe organizers I would recommend an over the door organizer. It keeps shoes off the floor, keeps pairs together, and makes it easy to see what you have.
Canvas bins with lids are my favorite. Most don't come with lids which I don't understand. But these will keep out the dust, let the items inside breathe, and don't contain plastic. It's a win win for everyone!
A stool close by, or in the closet is a must because it allows you to think "up" in your closet. Most people don't utilize the space between their shelves and ceiling, because the space is hard to reach. Having a stool makes the space easy to reach and easy to access.
At-A-Glance Closet Organizers are not essential at all, but for the person that has trouble setting boundaries for their clothing it is a nice tool.
I looked for these Closet Shelf Dividers at the local store and could not find them. So, I don't know about how well they work on your shelves, but I could imagine the possibilities for their uses. Not only for towels, sweaters, or other clothing, but maybe in your pantry to separate your can goods and such.
For kids closets this is amazing! Double Closet Rods allow them to not only to hang up their clothing, but get dressed without pulling a stool up to reach their clothing! This maximizes the ultimate amount of space up and down while keep the floor free!