Meta Search - Search your Desktop, Google Drive, Dropbox, Gmail, Evernote.
TagSpaces - TagSpaces is an open source platform for personal data management. With TagSpaces you can manage and organize the files on your laptop, tablet or smart phone.
Vaultedge - Access all your documents in one place.
Tabbles - Tabbles use tags to organize and find files along with your colleagues.
Google Cloud Search - Search across all your company's content in G Suite.
allTags - allTags is a free, tag based file management application.