The kernel modules of BlueZ are included in the Linux 2.6 kernel series. It is always a good idea to use the latest stable kernel. Kernel modules are in the Linux kernel since 2.4.6 release.
User Space BlueZ Package
To use the latest Bluetooth kernel modules is one important thing, but without support from the user space these modules are useless. For the minimal functionality the bluez package is needed.
For some Linux distributions pre-compiled binary packages exists. Other distributions picked up BlueZ and provide their own packages, so it is always a good idea to check the package lists of the distributions.
Source code is also available in the GIT repositories at kernel.org.
The 2.x , 3.x and 4.x series of libraries and packages are deprecated and not supported anymore by BlueZ developers. If you are using them please update to the 5.x series. The download link is only provided for reference.
bluez-firmware also figures in the deprecated category: