What tools can I use to write AS3?

I’m sure this has come up many times but my searches are netting me zip.

I’m trying to learn AS3, but I have no interest in the rest of Flash, I just want to write actionscript files, compile them, and be on my way. As such Flash CS4 seems completely overkill. I’d rather have a glorified text editor of some sorts.

Just kind of curious what’s out there. Open Source would be great, but it’s not going to be a deal breaker. This is for OS X.

Thanks!