I often get asked the question about Revit Add-Ons. Few people have the time to play around with new toys when they're not sure whether or not the time spent monkeying around with it will pay off in the end. Fortunately for you, I'm the guy who doesn't mind wasting my time testing these out. For anybody that's seen my ribbon in Revit, they know that I've looked at my fare share of add-ons. So here's a short list of some of the tools that I favor. I could go on and on because their or so many, but I'll keep it short enough to keep you awake.
1) Bluebeam Revu CAD - The key functionality that makes this tool worth it is the 3D PDF. If you've ever sent 3D files for others to review, the first response is usually "Do I have to install software to see this." In this case, the answer is no. Anybody with Acrobat Reader can view these files. The 3D PDF retains many of the Revit properties, objects act as layers that can be turned on and off, and the model can be fully sectioned, all in an easy-to-use format.
2) Kiwi Codes Family Browser - Anybody who works in Revit long enough understands that family management can be the most time consuming task in daily workflow. This tool provides a a nice graphic tool palette interface with thumbnails that are linked directly to your company standard files in Windows explorer. When a file is added, it is automatically populated into the tool palette for all to use. Organize it however you like. Its easy, intuitive organization at a very reasonable price. They also have a wonderful set of bonus tools.
3) COINS Auto Section Box - This is a very simple and functional tool for everyday use. Select an element, the apply the auto section box tool and it will create a 3D view with a section box around that element. There's no better way of resolving issues in a particular area very quickly. It's one of the top rated free tools in the Autodesk App Exchange.
4) Schedule Sync - Provides the basic functionality of exporting schedules directly to Excel, allowing you to modify the fields, then re-importing into Revit to update the data. Its a free app and well worth placing in your tool chest.
5) Tally - This is an app that is newly available, but has enormous potential. It's a building performance analysis tool that focuses on Life Cycle Assessment (LCA). Developed by KieranTimberlake, PE International, and Autodesk. I haven't played with this much yet, but I'll make sure to make the most of the trial. I hope its a game-changer.