Since I've seen many projects and therefore worked with lots of different tools, I put together a list of stuff I appreciate.
ReSharper, for faster coding in C# and consistent coding style. The one plugin I use for Visual Studio which makes coding (and refactoring) really fun.
Jira, to work agile in a bigger team. Even works for small teams. It is web-based (a good idea is to host it directly at Altassian to not worry about updates), so no need for an installation on each client. Became much better in matters of response time, so the disadvantage to a desktop client became less and less a problem.
Confluence, a central place for ideas, designs and other documents of a project. You just don't want to mess with Word documents on documentation heavy projects.
Jenkins, because you don't want to deploy your project every day (or more). From a certain team size you'll need regular versions of your project (continuous development). So setting up a build server once to take that work saves you a lot of time. Jenkins is open source, free and well tested.