I had a Treo from Cingular, it had absolutely no restrictions on installing programs. The preferred way to do so was generally by HotSyncing them from a desktop, but that wasn't any sort of developer mode, it was just how things were designed to work in the ecosystem (you wouldn't want to wait for the download over the air anyway).

There were a ton of aggregator sites for software with reviews, or you could just search the web for it, it was pretty easy.