SO searching around stackoverflow I found Intents List: Invoking Google Applications on Android Devices which tells me how to use intents to dial the phone, look at a map, or look at a streetview.
It does NOT tell me how to use intents to access the contacts lists, the calendar items, email, text messaging, media player, camera, bluetooth, or any of the other apps which exist on virtually every android phone.
It CERTAINLY doesn't tell me how to access with apps things like ES File Explorer, Dropbox, facebook, google voice, or any of these other very useful apps many of us download for free and might LOVE to automatically use from apps we write.
If any body out there knows how to find the "API" of intents for accessing these various things from apps, it would be so great to be able to peruse these lists.
PS I realize I can LAUNCH any of these apps with an intent, I know how to do that. But to automatically upload something to dropbox rather than bringing up Dropbox's entry screen so that I can manually try to guess what my app wanted to upload and then upload it by hand, obviously automatic is better or we wouldn't be programming phones.
This was the biggest list of intents I could find. I use those quite often. All the intents listed here are the ones that various apps let developers access to. If youre looking for a specific app's intents, then they would list that on their developer forums/site. Apart from this I dont think there are other ways of knowing which app provides which intents.
EDIT: You can find another list of intents here.