I just signed up for a free account and tried out Pivotal Tracker and I must say I agree with the testimonials I saw on the site. Having been used to Basecamp for a couple of years, it was a very different experience and philosophy but I got a hang of it pretty quick. Here the main things I like about it so far:

• Its Free
• Its web-based
• Its puts everything for everyone on the project on the very same page. I am big fan of Excel and use it very creatively for a ton of things and love to see everything on the same screen.
• Its JS based and super fast like Gmail
• Its realtime so everyone on the team can see the changes happening in real time
• It took me 2 mins to get started and understand the concept of stories, icebox etc. by just clicking around. Easy to get started.

The testimonials on their site is what made me try though. Some pretty impressive people giving it a thumps up:

“We are using Pivotal Tracker to manage all of our new web apps under development, this thing rocks. It has a very nice workflow and has really helped us know what other team members are working on and to know what our velocity of development is. ”
- Ezra Zygmuntowicz, Founder and Senior Fellow, Engine Yard

“Pivotal Tracker is a fantastic product. We’ve become a more efficient development team since we started using it. And even better, the management team gets on-demand visibility into my team’s priorities and accomplishments. ”
- Jim Gochee, VP Engineering, New Relic

“Tracker is the ideal story tracking tool for truly Agile/XP environments. Its slim feature set keeps you focused on stories and velocity, rather than distracted with nuances and overloaded features and process. Being Agile is about keeping energy focused on writing code, not burning cycles on tools that turn out more and more process. Tracker’s sweet spot is its concise, lightweight, only-what-you-need, user interface. I’m a fanboy.”
- Jud Valeski, CTO, GNIP

Share your comments here and I will be sure to share more on how we leverage it internally.