TeamCity

TeamCity Continuous integation — automated scheduled project compilation and build tasks, designed to highlight integration problems early on. TeamCity by JetBrains is one of the best CI tools around.

TeamCity is free for small projects, supports plugins (there are other 100 of them already), can scale infinitely, has a nice, easy-to-use user interface (which is typical of JetBrains), very easy to use, despite massive number of features.

Similar tools, such as Bamboo and CruiseControl, although great products themselves, are harder to use and have limited functionality when compared to TeamCity.