Target Process is a great Agile process management and iterative development tool. It is amazing for several reasons.

Firstly the people who make the tool clearly work within a very agile environment. It is clear from their blog that what they do on a day to day basis is represented in the sheer coverage of the tool.

It is a tool that is priced very well (ie it's super cheap -especially if you let them host it). Unlike some other tools which cost literally thousands (thinking of a large test tool provider here whose name starts with M), Target Process is cheap and yet it does everything a true Agile developer, PM, tester, customer could want it to do. It can manage whole projects and does so in a very cool way.

Target process has nice features such as Task boards which can be used for distributed teams. It represents stories and sprints (iterations) really well too. If your team is distributed around the world or even in multiple offices then it's a great tool for you.

It's very simple and isn't overly busy like many other web based Agile tools.

It easily fits into Scrum or XP models, as well as FDD and presumably any other Agile or iterative process model. You can even change the workflows and terminology associated with any given process.

It's well designed (Ajax based web browser interface), its quite fast and it is very slick. Not dull and depressing like some others. It also has a really cool reporting capability.

Overall I can't recommend this tool enough. I've used it before on projects and I even use it for small background work for my company, Supple Technology Solutions (UK).  Its even something you could adapt for personal endevours such as the "Getting Things Done" method ... and it' not so expensive to make that seem ridiculous. 

Version 2.7 Is out now and has been improved even more. In fact another cool thing about this product is that they update it very often, usually with very clever features. You can tell that this product's features are motivated by developers who work in an iterative environment and not marketing and product managers . Let's hope it stays that way!