I use Basecamp and love it. It works wonderfully for me, but the best part is that it works for my clients.
My clients are not always computer savvy. They may range from a project manager living life through a Gantt Chart to an install lead terminating wires in a rack room. No matter the level of computer knowledge the person has, Basecamp keeps the communication on track.
The system is simple (not bloated), clean (not flashy), and does not assume anything (that you have a BS in Computer Science).
One of the major reasons that the system works so well (I feel their are many to choose from) is due to language.
The other morning this images pops up for me in Basecamp.

This simple flash message / alert brings me joy for a couple reasons.
(It came from a human) It says “We’re looking into the issue right now” and “We’ll post and update when its all fixed up”. This gives a comfort level to the user.
(They are always on top of it) This message also tells me that I can rely on their system. It says to the user that they have an issue on their end, they are on it, and they will let you know when its fixed. You may have never noticed this issue, but they did.
Another message that came through is this one:

In the same vain. This information matters to people, especially when you use it for business productivity. This is not Facebook, we are not looking for fans, we are getting shit done.
The system guides the user using language and not generic form fields, and does it very well.
Here are some more visual references.

This image appears if there are no messages yet for the project. Stacked full of verbs, a embedded video to get you moving forward, and a link to a screenshot based tutorial & tour page.
Look at that large green statement “Click me to start the video” in place of that ever expected right facing triangle that most of us know to be a play icon.
My lord the sales department can get in on this. (Sorry sales folks)
Here is another.

Verb, verbs, verbs.
Ryan Singer from 37signals gave a talk on “UI Fundamentals for Programmers” for WindyCityRails where he really drives this home.
Ryan says “No verbs is a UI smell” and I agree, and will push for this in every project I do in the future.