I like the way that it integrates with all of my devices. Google has gotten better at it since they started, but I picked Dropbox back in the day and have had no desire to switch. I also really like that Dropbox allows you to link your personal and company dropboxes. The team feature is a big bonus.
We’re not using a company-wide accounting tool now, but I personally use Expensify to track my expenses, as it makes it really easy to create corporate expense reports and track receipts.
We love them. Their service gives you a corporate number with extensions, and all of the extensions forward directly to your employees’ phones--either their cell or desk. When you’re small, it's an inexpensive way to gain the appearance of being big and more stable.
I’m big on transparency, and Trello is so good for that purpose: It's list-driven and easy to prioritize tasks, which makes it so easy to see and manage tasks. It's almost like having a sticky note board. Very visual. I also love that it's Kanvan-oriented. I liked Asana, but it just didn't have a way to view data in that format.