Some here use a spreadsheet to make inventory and / or want lists, others may use a word processor with tables inserted, and there have been threads about dedicated philatelic software programs.
Way back in my Palm Pilot days (when keeping track of customer, prospect, product, and order data on the road was really important) I began using HanDBase. It is a relational database that has forms, can perform calculations, has good filtering capability, and can synchronize data with MS Access. It is available as desktop software (Windows and Mac) and as a mobile device application (Android and the i-thingies), which can synchronize with each other.
This program might be of interest for maintaining stamp inventories or for always having your current want list readily available on the mobile device. I have found it very useful for recording financial transactions in the field, passwords (you can password protect a database), shopping lists, etc.www.ddhsoftware.com*** Moved by Staff to a more appropriate forum. ***