Over the past year, Trello has become one of the key apps/services of my operations. It's the place that I manage all work I've sent to others and is an excellent tool for teams and collaborators. I'm going to write up in detail exactly how I use Trello in the future but a simple explanation is that I've templated all the typical projects I send to my helpers and use Trello to assign the work and keep track of its progress.
If I had one gripe against Trello its that it has always felt, nearly exclusively, like a web app. There are versions of the app for iOS and Mac but they have always been pretty rough around the edges. I'd say that's still true for the Mac app but yesterday they released an impressive update for their iOS app.
New features include: custom fields such as dates and text, drop-down lists, and checkboxes, a better date selector, better linking throughout, the ability to add more than one attachment at a time (hallelujah!).
I use attachments often with Trello. If I'm having someone work on a document for me, I'll attach the document to the Trello card rather than emailing separately. The best new feature is the support for drag and drop, including attachments onto a card.
I'm happy to see Trello taking advantage the iOS platform features. I hope they put a similar amount of effort into the Mac app.