I recently tried corn silk, although I haven't tested the soap yet.
I started using silk (dissolved in the lye water) a few months ago, but again, haven't been using it long enough yet to be definitive about the results.
I have been using ROE & EDTA for a couple of years at least and can say without a doubt that soap scum is reduced and I don't see DOS in my soaps hardly ever.
Sometimes I have used SL, but don't really see any long-term benefit, only use it now and then when I think of it so I can use up the bottle.
I have used salt and sugar more often than most other types of additives. Salt to harden the bar. Sugar to enhance bubbles. But with a longer cure, most of my soaps don't seem to need either, so have cut back on that habit, too.
When newer to soapmaking, I used Aloe Juice for my lye solution in many batches of soap, but stopped using it mostly because it was just a hassle having to refrigerate or put into cubes in the freezer.
I have used honey, beeswax (I like beeswax), lanolin (I also like lanolin), maple syrup (too much can make soap spongy), agave syrup, colloidal oatmeal (with a long cure this gets scratchy to my skin), dried tea leaves (only for foot soap though, due to scratchiness), and various other things.
I liked trying out different purees like papaya, carrots, tomatoes, etc. And I've experimented with other botanicals as well, spinach, parsley, other herbs and spices, too. Some for color, some just to see what would happen. But none have become a favorite that I use a lot in soap. Most were more of a novelty than anything else.
Now and then I still use different milks in soap, but don't really see such a great benefit, so it's not really a favorite for me. I like to drink buttermilk, but sometimes when I have it, I'll toss some into a soap for the heck of it. Sometimes I'll think, 'why not open a can of goat's milk?' (I still have maybe 3 cans in the cupboard) and use that until it's gone. But I don't really feel a huge benefit from milks, so not a favorite. But it makes nice gifts as many people do like the idea of goat milk soap.
The one additive that I really felt made a difference in the soap (and I do admit, this might be my own personal prejudice) was roses-infused oils. I gather the rose petals from my rose bushes every season and dry them for potpourri. But I also infuse some of them in oil and use the infused oil in a small batches of soap. Besides nice label appeal, for me I do think the soap feels different to my skin, even if maybe it's all in my head. Although I do have to say that I did once use one of the soaps without forethought and while using said to myself, 'This feels like my rose-infused oils soap', so maybe it is more than in my head and maybe it is more than a nice label appeal.
