Like some other people here, I really love Hudson.
It is extremly easy to install (it's just a war file to deploy), to configure and to use.
In addition, it offers a lot of functionalities that are not always available on others systems (build matrix, master and slaves, security on server or project level...)
The number of existing plugins is quite important now, and creating its own plugin is not that hard...
To finish, this application evolves really fast: we are currently on the 257th release and they made more than 100 releases since one year!
For french people, I wrote a quite complete guide to use Hudson here.