Send to Dropbox - Email attachments to a Dropbox folder
Sortd - Rated the #1 App for Gmail
Get My Receipts by cloudHQ - Organize your receipts in one click
KanbanMail - A Kanban board for your emails.
OneBox.io - Sync e-mail attachments to GDrive, Dropbox, Box, or OneDrive
Trello - Infinitely flexible. Incredibly easy to use. Great mobile apps. It's free. Trello keeps track of everything, from the big picture to the minute details.