Sortd - Rated the #1 App for Gmail
WiseStamp - WiseStamp allows you to easily create and implement slick and unique email signatures for your business.
KanbanMail - A Kanban board for your emails.
NEWOLDSTAMP - Email signature generator
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.
Sigstr - Email signature marketing