I'm using lastpass' 'Secure Note' feature. There are some Secure Note templates for specific purposes, but there is also a generic one. It can store free text, which I find sufficient in many cases - no need for custom fields. if needed, I use something like this as a free text, for example:
Secure Note Name: S/N: headset 01
Secure Note Name: Door: Lab
So far this works for me. Note, the beginning like "S/N:" (stands for 'serial number' here) or "Door:" in the Secure Note 'name' field allows me to search for a specific types of Secure Notes; e.g. if I'm looking for locks, or doors, I'd search for 'Door' and the relevant secure notes should be listed as a result.
It can also store attachments, that I find useful in some cases (add a small image of something, or add a router configuration backup file). A Secure Note can also be additionally protected with the 'master password required' option if desired, which might be useful in case of very sensitive information.