Yes there are adapter, but the question is do the other mp3 players allow one to control the device from an outside source. I have seen a few mp3 players, and they did not (the ones I have looked at, I am sure there are some out there that don't fit this) a way to control the device from an external accessories. I will say for my iPod I have a device that when plugged in adds a IR port so I can use a remote control on it. I am talking about add one ability, that what made computers nice, you can get one and add things to it, would you buy a computer if you could not add or replace the hardware? I know you can not really replace the hardware on the iPod (well most people don't think you can, I know how to replace the harddrive, and I have replaced the battery in my 2 year old iPod, but you can do that for any mp3 player) but with the iPod it was put together in such a way that things can be added on. How many accessories exist for other mp3 players, I have for my iPod 6 accessories, and only one was made by Apple (I really love my iHome)
I bought a Creative mp3 player last year. It was cheap, the interface isn't very good, but I can control it from my computer. No remote control or anything. I have an fm radio transmitter for it but it didn't work very well. I think it was for the iPod.
I still have 40GB of storage and it plays just fine. It's going to be my main source of music in my home when I get my stereo hooked up. I think the FM transmitter will work well inside the house. Then I can control all from the computer.