Add-ons help make software better or adapt to your style of work. Sometimes the size of a teeny tee shirt pocket, add-ons are accessible as a desktop widget or from a browser on multiple devices. And a secret about add-ons--it is a telltale sign that many people like the product the add-on is developed for. Over at 37signals, maker of Basecamp, the popular project collaboration tool, there are plenty of add-ons produced by third party developers.
If you're not familiar with Basecamp, you may want to read this quick introduction for project collaborators. So let's experiment with a few to give you the gist of Basecamp's add-ons.

EazyBI for small or any size business takes the complexity out of business intelligence. Integrating your Basecamp data into EazyBI is done using three easy steps - Dashboard, Source Data, and Analyze. EazyBI is also a handy tool to use for importing data in CSV format from other applications. The free price tag can get you started and when you're ready, the average price is $50.00 per month.
Do you call and meet people on Skype? I discovered viaSkype, a free service connecting activities between Skype and other online applications, in particular, Basecamp. ViaSkype is unique in updating your Basecamp activities while you're logged into Skype. In meetings, many people, including myself, like to take notes on a laptop or tablet, so why not enter activities right into your project collaboration tool.
For Freshbooks, the web-based billing application getting good write-ups is now improving through its' ability to record project expenses against a project as an add-on to Basecamp. Essentially, Freshbooks will invoice or track time for your Basecamp projects. Clients love working with Basecamp and will be happy about Freshbooks monthly price, from free to $39.00 and up based on additional staff who access the account.
Take a look at other integrated tools to work with Basecamp. Yes, so many apps vie for our attention. Keep your eye out for other Basecamp add-ons, too--the list is growing.

Although more designed as a task scheduling addon for Basecamp, Workstack can also maximise Basecamps native ability in recording time taken on tasks.
Thanks for the info on Workstack, Spencer.