How to find software solutions for Linux on System z

There are multiple sources for finding middleware and solutions for Linux. As the web sites get updated this may change - let me know then I adapt the description.
Please note that none of those catalogs is complete, they are all snapshots. 

IBM Global Solution Directory (GSD)
Here you can create complex searches. To select Linux on System z select "IBM System z (Mainframe)" under platform configurations and then all the operating systems you are interested in. Finally click on "search" without entering anything else.
 
IBM Software product compatibility
The most important reports the operating system reports. When generating searches please be aware that their are two different entries for RHEL. The base one has RHEL2 - RHEL5 whereas the one titled RHEL server has RHEL6.
SUSE Enterprise Linux server is all under SLES.

IBM System z page
This page lists the ISV solutions updated in approximately the last year.

Red Hat Product Catalog
All the products registered with Red Hat. After you've searched for a specific product you can check the platform and see where it's running.
Note that Red Hat is still using the search term "zSeries" for referring to "System z". So you may want to try this search term as well.

SUSE Linux Enterprise Software Catalog
SUSE has a catalog that can be searched quite easily. Click on "advanced search" and select "IBM System z" as the platform and you get all the vendors offering products for the mainframe that registered with SUSE.