Any mac application that works for 10.x will work for 10.5.6. One thing you should get used to is that Apple updates their OS every month or more frequently, and the updates are completely free. So the version you have is inconsequential to the app you want to download.
There are many resources for downloading apps.
The first, and most efficient, is google. Google the name of the PC application along with the word “mac”. This will help you find the Mac equivalent of any PC application. For example:
“Nero burning rom mac”
Next, you can visit software repositories.
www.apple.com/downloads/macosx/
www.softpedia.com/mac
www.versiontracker.com/