Latest official release: Pak128 1.4.3 for Simutrans 99.17.1
The latest official release of Pak128 is version 1.4.3, which is compatible only to Simutrans 99.17.1. This version will not work with simutrans 100.x! You can get it from Sourceforge (http://sourceforge.net/project/showfiles.php?group_id=196471), where you can also find the executables of different official Simutrans releases, for the different operating systems.
Unofficial pre-release of Pak128 for Simutrans 100.0
There is also a pre-release (pak128_1-4-3-1pre) for the next Pak128 version, which can be used with Simutrans 100.0. Savegames created with this release are not usable with Pak128 1.4.3! You can download it from the aforementioned Sourceforge link.
Nightly builds of Pak128
On the Nightly page (http://simutrans-germany.com/~nightly/simutrans/index.php?lang=en), you can find pak128 nightly builds, which will run with the latest executable, which is also available from that page. Note that these Pak128 builds are not official releases, and savegames created with them are not usable with Pak128 1.4.3!