I worked in smart phone/PDA support before the iPhone came out. I was actually working tier 2 for a Sprint call center vendor when the iPhone was announced. There was a ton of third party support. The big three at the time (this was in 2006) were Blackberry, Palm, and Windows Mobile. Blackberry didn't really have any third party software, but they got by with their BES (Blackberry Enterprise Server) integration. Palm and Windows mobile both had lots of third party software available. I did a quick check to verify and Wikipedia says that Palm OS had over 50000 third party packages in 2008.

Apple, and later Google when Android got better (who remembers the pre 1.0 releases? They weren't great) changed the face of mobile, but they didn't invent smartphones, or the idea of a third party app ecosystem. They made it much better, but Apple didn't even originally support third party apps. Steve Jobs thought that everyone who wanted other types of apps would just use web apps, but that hasn't really happened due to a number of factors (partly OS manufacturers locking down which APIs are available, although I haven't worked on that side of development for a few years so it might have changed).

edit: I forgot about Symbian. I worked in the US and it wasn't really that common, at least I never talked with a customer that used it.