Antibacterial Ointment - I hesitate to call it optional, it almost feels irresponsible...however, keep in mind that the Albergues have first aid supplies and that there will be pharmacies along the way, not to mention many pilgrims carrying tons of medical supplies.
Here are my reasons:
1) I was astounded and much annoyed at how much extra room and weight all my fear induced medical supplies ended up adding to my pack. I'm a pilgrim who also ended up making a trip to the hospital in Léon, so I wasn't just "lucky" in having no need for antibacterial ointment.
2) I brought a tube of antibacterial ointment. Because it was in an aluminum tube it got bent and began oozing in my backpack at some point. It wasn't nice finding an orange goo in my pack at the end of the day. If you work hard on blister prevention then you are not likely to need it. I did buy another tube along the way, this time in plastic, but never used it. That said, please use common sense: if you borrow a needle to pop a blister, sterilize it, and then use the ointment.
Tip: If you decide to bring some, squeeze 3/4 of it out before you pack it.
Belt - I lost quite a bit of weight walking every day. My pants began feel as if they might fall down. I tried holding them up with string, and pinning them with saftey pins. Don't know that I want to carry a belt, so maybe there are some zip-off pants out there with adjustable straps in the waist. Thank goodness for womanly hips.
Cell Phone - Cell phones were annoying for those of us trying to rest and in some cases while walking. I wouldn't be above bringing one, however. In fact, I considered trying to blog with one along the way but decided I couldn't afford the equipment I would need. It would certainly be good in an emergency. So, bring what you must, but use common sense, and take fellow pilgrims into extreme consideration. Personally I found it liberating not having technology interfering with my journey.
Digital Camera - I brought an Olympus [mju]:410 (That's what it says on the back of the camera as model). It is a nice size, I wouldn't want to carry anything much larger. It is built for more rugged use which I accidentally tested one night when I dropped it from a top bunk bed. It is water safe, you can take pictures in the rain. I did have problems with it however. The main button for taking the picture is too hard to push without moving your picture. Even in full daylight I sometimes got a series of moved pictures. This camera has a default setting that sets the flash off in bright daylight unless you happen to know how to get the camera to remember your settings. So, the user interface is not transparent and is very frustrating. Another mystery was the time I left it inadvertently recharging all night and the next day when I opened the camera lens cover it started making all sorts of sounds for every interaction. Sliding the lens cover off produced the "sprinkling-of-fairy-dust" sound, taking a picture produced the "high-low-high-come-here Rover-sliding whistle" noise. It took quite a while to figure how to shut it up and off. I would chose a model with a larger display area. It wasn't a problem to recharge the battery along the way in the Albergues. I carried a total of 8 memory cards and I don't think I used them all even with taking approximately 800+ photos.
Are there no cameras out there for left-handed folks? Seriously, I'm asking.
Guidebooks - Try to take only one. I brought 4: 3 guidebooks, a Spanish phrase book, and my journal. (I have a little disease that compels me to bring the National Library of Deborah whenever I travel. Yes, even when I know I'm going to be walking with said library on my back for 500 miles.) I ended up sending the Fisterra guidebook, my German guidebook that I wasn't using much, and the Spanish phrase book ahead to Santiago. I have put the guidebooks under optional because there really isn't much chance of getting lost on the Camino Francés from St. Jean Pied de Port and Santiago. Guidebooks do help you calculate how far each day you will be walking and should your body not be able to walk as far as planned you can then recalculate. I think it is entirely possible to go without a guidebook. You will be surrounded by pilgrims in the Albergues who will be daily quietly, yet ever so slightly nervously, calculating their next day's travel, should you be in real need of a quick look at a guidebook.
Journal - my favorite journaling tool is the Moleskine lined notebook. Especially appreciated is the pocket in the back, which I used for postcards, pieces of paper of telephone numbers of people I met, or email addresses, etc. I just can't imagine a better size or better designed product for journaling on the Camino.
Magnesium Tablets - In Germany magnesium tablets are the preferred muscle cramp preventative. I don't know if they helped me or not because I still got muscle cramps, but I took some along.
Sarong - might be useful if there is room, but would be very low on my priority list. I can see it as something to put around your shoulders as you are getting dressed in the bathroom, as it can sometimes be chilly. I saw some people using them to cover the pillows provided by the Albergues. Another use is something to sit on during a break. You have to weigh how many functions something like this can have before seriously considering carrying it.
Suction Cups with Hook - quantity: 2? I almost had the opportunity to use one of these on the Camino but lost the one that a pilgrim gave me. It was challenging at times to take a shower and not get your clean clothes wet, or your towel. Having a place to set a bottle of shampoo or your cake of soap was mostly nonexistent. This is where if you had one of those hanging organizer bags, and a suction cup you might help yourself out. But...it needs testing and hopefully I'll be able to report back on this at some date in the future.
Travel Vest - I brought one and ended up sending it on ahead. I wanted one because of the various pockets that I could use for carrying things such as snacks, my camera, etc. It just ended up being too hot to wear.
Tweezers - Handy for a variety of uses, but optional.
Watch - I didn't bring a watch. I didn't need a watch. I admit I was pretty paranoid in the first week about not having a watch. A watch with an alarm function could have been practical, but then again, if you are sleeping in the Albergues, unless you are an extremely heavy sleeper you won't have any trouble or choice being awakened fairly early every day. Don't let this deter you from staying in the Albergues. It was really one of the best aspects of the Camino.
Another reason to leave it behind: you can only walk as fast as you can walk and no one is timing you. You will eat when you are hungry, rest when you are tired. Letting go of time can be very liberating. And just when and where are you going to get that kind of time again?
Writing implement - for journaling. I nearly went crazy because the ink pen I had brought was "fudging" on me at one point. Bring a writing implement that will be a pleasure to write with and not a pain.