These days my Bread Journal gets the most use. In this I keep 2 basic slow-rise recipes. I also record every batch and loaf made. My bread instructor (Brad Williams) suggested keeping a journal to keep track of successes & failures. It comes in handy to remember how much water to use with certain flours. Brad also suggests to record weather because this can affect the dough. Keeping a bread journal has helped me to try again after failures by varying the amounts of certain ingredients.
My Knitting Journal is where I used to detail projects yarn, pattern, needle sizes, etc., however, since discovering Ravelry, project details are now online. This paper journal is mainly a place where I glue yarn labels & samples. I also glued a paper ruler (inches & cm) on the inside cover to measure swatches when travelling. There's always a printed glove pattern in the back pouch for when the Internet is not accessible.
My newest journal is a Soap Journal where I record soapmaking adventures. I keep track of the scents, colours, and bits which were used in the soaps. I only need this one 2x/year so it's usually difficult to find.
I really enjoy the tactile feeling of writing on paper and looking back at past entries but sometimes it's just more practical to use a digital version. I wonder whether paper journals will soon become a thing of the past. Luckily, bookstores still sell beautiful notebooks. My favorites are Moleskine Grid Journals. Project journals, whether high or low-tech, are excellent auxiliary memory devices. Their real purpose is to write notes to my future self to improve on what I just did.