The humble A-Z tabs... we have come to think of them as just for sorting our names and addresses or our contacts. In a recent FFAF people freely admitted that they don't keep their contacts in their Filofax.
I was surprised by this, but I can see some people's reasoning for relying more and more on their phone which hopefully they have backed up on their PC or in the 'Cloud'
So some other ideas on how you could use the A-Z tabs which I'm sure you have still.
I use my 'Internet Site Info' sheets for recording Site, Username, Password details on and these are stored within my A-Z tabs based on the Site name.
I don't record the full password. As I use several similar passwords I might only record things like O.......79 the number of dots does not however match the number of letters. So anyone finding my Filofax would not have access to any of my passwords.
I tend to use similar letter patterns but just change the upper and lower case elements, so another site using the same phrase as above might be recorded as o.....U79, it just serves as a reminder to me.
When I was learning to use the command line version of the Linux Operating System. I had a series of note pages within my A-Z tabs to record the details of various commands and how I specifically needed to use them on my computer.
In my old job I was always being tripped up by abbreviations, military people have an annoying habit (to me that is!) of reusing abbreviations in common use, but for something completely different. Reading their reports was becoming a total headache trying to find what they met by NEC or TCM etc So I started another A-Z index of abbreviations to be able to look them up faster.
Our Philofaxy community has grown considerably in the past year or so, I'm thinking of using the A-Z tabs to create a 'User Index' so I can cross reference Comment Names vs Twitter Names vs Real Life Names... my poor grey matter tends to be a little forgetful at times when it comes to names!
If you use a Filofax to store recipes you could use the tabs to divide up the recipes by name or type.
Or if you keep your own personal reviews of books then may be use the A-Z to separate our your reviews by Author name.
I'm sure you can think of some more uses besides the ones I've outlined.