Web-Based Project Management Tools?

zbignew asks: "I am working on a Web-based project with four other people spread out at various geographic locations around the world. The work is picking up and we are hiring again and the new people are also likely to be at new locations. To help keep workflow organized we are looking for some kind of a tool to track tasks, issue resolution and things like that. It must be Web-based. Are there any good ones out there? We will pay if we have to for quality."