Samsung BlackJack SGH-i607

Everytime I try to download software for my blackjack, whether I do it on my computer and then send it to my blackjack, or if I download it from the blackjack itself, it never works. Either the CAB file won't run, or it will successfully install, but the program won't run. The only software that works is stuff that you download from the Cingular/AT&T mall. There's a lot of cool free software out there on the internet that says its compatible with WM5... so is it the Blackjack? I was so psyched when I got this phone so I would hope it is capable to run the software I want to run on it. Can anyone tell me what the problem is, and how to fix it if it is possible?

Thanks!