My two main productivity apps are Todoist for Mac and iPhone and Microsoft's OneNote for the same platforms. I've found these two apps to make a nice pair in my daily job at DIRECTV, as well as for my side projects (running Bad Timing Records, writing for AbsolutePunk, and any personal work / life things I need to manage as well).
Todoist (iPhone, Mac) is very nice task list. I have projects set up for DIRECTV, personal work (which encapsulates anything I'm doing on the side, including AbsolutePunk stuff) and Bad Timing Records (which is divided into sub-projects of Tasks, Timeline and Production). I also have a project that serves basically as a calendar for things my girlfriend and I have going on after work -- I use that to track the shows I'm going to and any other plans we have (podcasting, playing basketball, work-related meetings after hours, etc).
The main plus of Todoist comes in its sharing capabilities. I share the Bad Timing Records project with Zack and our publicist / finance hero Emily. I share the calendar project with my girlfriend, Katie. This enables you to assign tasks to people -- I can create a task for Zack to update an item in our webstore and assign it to him with a due date on a particular day, and he'll see it pop up in his feed.
There's also a nice way to create sub-tasks, which I use for big projects at DIRECTV. If I'm working on a project to refresh a marketing page, I'll have that project exist as one task with several sub-tasks nested inside for what needs to happen. I rely in to-do lists in an extreme way; if something doesn't pop up in there, it basically isn't going to get done.
I complement Todoist with OneNote (iPhone, Mac) for the purposes of note-taking and project tracking. For any project I'm working on, there exists a "page" within a broader "notebook" on OneNote. I'll do everything from writing album reviews to tracking huge amounts of integration changes I'm making at DIRECTV here.
I'm planning on doing regular recommendations for apps like this; these two seemed like the most logical to begin with given how regularly I use them every day.