wordpress.org codebase

You can try out the api.wordpress.org.

Take for example the secret MP6 plugin:

http://api.wordpress.org/plugins/info/1.0/mp6.xml

The current version is given by:

<version type="string">
<![CDATA[ 0.8 ]]>
</version>

and it looks like the previous versions are listed in the compatibility tag:

<compatibility type="array">
<array key="3.4.1">
<array key="1.0.0">
<integer>100</integer>
<integer>1</integer>
<integer>1</integer>
</array>
<array key="1.0.1">
<integer>100</integer>
<integer>1</integer>
<integer>1</integer>
</array>
<array key="1.0.2">
<integer>100</integer>
<integer>3</integer>
<integer>3</integer>
</array>
</array>
<array key="3.4.2">
<array key="1.0.2">
<integer>100</integer>
<integer>1</integer>
<integer>1</integer>
</array>
<array key="1.0.5">
<integer>100</integer>
<integer>1</integer>
<integer>1</integer>
</array>
</array>
<array key="3.5">
<array key="1.0.5">
<integer>100</integer>
<integer>1</integer>
<integer>1</integer>
</array>
</array>
<array key="3.5.1">
<array key="1.0.6">
<integer>100</integer>
<integer>2</integer>
<integer>2</integer>
</array>
</array>
</compatibility>

You can compare it to the SVN versions given here:

http://wordpress.org/extend/plugins/mp6/developers/

You can also get json or serialized php versions:

http://api.wordpress.org/plugins/info/1.0/mp6.json
http://api.wordpress.org/plugins/info/1.0/mp6.php