Is WindowBuilder Pro already installed in Eclipse?
WindowBuilder seems to be the best gui designer at the moment, since it can parse and work with any manually written code, it doesn't require any additional files(xmls etc). And it always tries to make the smallest change possible to the code.
WindowBuilder is now included in Eclipse IDE for Java Developers.
eclipse-java-kepler-SR2-win32.zip
UPDATE 2019
Again WindowBuilder Pro is no longer included in any of the latest Eclipse versions. So you need to add it as was shown in the initial answer below.
It is also strange, that WindowBuilder Pro is not available when searching Eclipse Market place (checked with the latest as of now version of Eclipse For Enterprise Java Developers version 2019-09) despite the fact that the product's website is available.
But at least the related repository URL for the latest version of WindowBuilder Pro is now much compact and intuitive:
http://download.eclipse.org/windowbuilder/latest/
UPDATE 2014
As Sean Lynch noted in his answer WindowBuilder is included in Eclipse IDE for Java. Actually, it was already included there since version 3.7 (Eclipse Indigo), which was released in year 2011.
Note that currently, it is included by default only in the version for Java Developers, NOT in the version for Java EE Developers, which is the most downloaded and wanted version. So for the Eclipse IDE for Java EE Developers you still need to install it yourself.
Old answer
It is not installed by default in Eclipse.
The new WindowBuilder Pro home page is here:
http://www.eclipse.org/windowbuilder/
On that page, in the "Current Status" section on the right, it says (emphasis mine):
"Welcome to the new WindowBuilder homepage!"
How to install WindowBuilder in Eclipse.
1) Copy the correct link for your version of Eclipse from the Update Site column (Release Version) on the download page:
2) In the Eclipse main menu choose Help > Install new Software...
3) In the appeared window click Add... button:
4) In the appeared dialog box, in the Name field write some descriptive name (like "WindowBuilder Pro") and paste the correct link (see step 1) in the Location field. After that click the OK button:
5) Select all the checkboxes that are going to appear, then click Next button and install WindowBuilder:
Another way to add a link to the list of available Software Sites:
in the Eclipse main menu choose Window > Preferences, in the preferences window choose Install/Update > Available Software Sites, click Add... button and go on as described above.
As for the better alternative, well, seems like WindowBuilder Pro is the most preferred: Best GUI designer for Eclipse?
Hope this helps.