The current Labs version is pointed to from the front page of the Developers Wiki (see the Alfresco main page). There are currently installation kits for Linux, OS-X and Windows, some of which are pre-configured with a Tomcat web server. The full package is around 300MB, so it can take quite a while to pull down.

There are a variety of other packages on offer in the Labs distribution, most interesting of which, are probably the plugins for Microsoft Office packages. These are currently found at the bottom of the downloads page.

Windows Complete Installation

Download the appropriate Alfresco-Labs-XX-Full-Setup.exe package.
Open the .exe file
Select the language to use (default is English).
A confirmation dialogue appears, click 'Yes'.
InstallJammer Wizard appears, click 'Next >'.
Setup type dialogue appears, select 'Typical' and click 'Next >'.
Installation folder dialogue appears. The default for Windows is C:\Alfresco. Click 'Next >'.
Review page appears. Check your options and click 'Next >'.
Installation starts. First the main components are installed, then, if required, Java (JDK) is installed.
Database selection dialogue appears. You can either use the built-in 'Derby' database or an already installed and running MySQL database. Derby is recommended for evaluation and training use, although it will need to be replaced by a heavier duty database for production work. Make your selection and click 'Next >'.
Final page is shown. By default, both the 'View Readme' and 'View Alfresco Labs Wiki' are checked. Uncheck them unless you really need to view one or both. Click 'Finish'.

Linux Complete Installation

This is somewhat more convoluted than the Windows installation as a number of pre-requisites are required before installing Alfresco itself. Please note that this process is based on the Redhat Linux distribution, in this case, Fedora 9. JDK and MySQL are required to make a clean install under Linux.

Check if you have the current version of the Java SE Development Kit (aka JDK) on your host. If not, download and install this first, from the Java Website.
Confirm that the variable $JAVA_HOME exists in your environment and that it points to the JDK's home directory.
Check if you have a current version of MySQL installed, if not, download and install it.
Download Alfresco-Labs-vv-Linux-x86-Install to your desktop. Note that the installer requires a running X-Windows session to run it.
Ensure that the Alfresco installation kit is both readable and executable by your user ID. From most desktop managers (Gnome, KDE, etc) you can right-click the icon and select Properties from the menu displayed. Otherwise, open a terminal and check the permissions by using 'ls -al'.

Alfresco in a Tin

You can pick up a ready to run virtual machine, with Alfresco pre-installed, from the Virtual Appliance Marketplace. Enter 'Alfresco' into the Search Available Appliances box to see a list of images.

You'll need VMware to run any appliance you download, of course.