Jenkins (formerly Hudson) is an open source continuous integration server and assists the development team by allowing us to easily integrate changes into our software projects from our source control with confidence. Jenkins can be automated to build, test and deploy any type of software automagically. Jenkins doesn't require bucket loads of learning before you start using it and it's very easy to set up. It's also super extensible and caters for a wide variety of programming languages/testing frameworks etc and there are a lot of plugins available - and I mean a lot.
Feedback is key. If Jenkins has any problems building, testing or deploying any projects it can alert the development team in a number of ways (e.g. through a GUI, email, twitter and many more). So as soon as the build or any tests fail we get alerted and the team react to fix those changes - it's never broken for long.
As you can imagine, once we made the effort to set up our software projects in Jenkins, it has improved our 'quality of dev life' dramatically. Automating the building, unit testing, integration testing and deployment straight to our staging environment for every check-in means we are more confident, less stressed and a lot more productive. Once happy with our staging environment we can one click an application release to live. So our application users are getting newer features faster than ever. Ignoring IDEs and source control, I'd argue a CI server is probably the next most fundamental tool in a developers tool belt.
Jenkins! How did we ever live without you?!