The paper sticky note--it hasn't gone away and you can buy them everywhere.
Paper sticky notes are inexpensive, ubiquitous, can be used by anyone, and are still an elegant means of organizing yourself or your team.
Any digital equivalent is likely to be locked in a particular web site or technology company's proprietary system (e.g. Google Keep, Apple Notes, Microsoft OneNote, etc.) If you are happy to use a locked system, then one of these might work, but it comes with all of the advantages and disadvantages of those ecosystems.