RE: ".. I just want an app/application pair that can quickly share links, notes, and pictures between devices, like iMessage."
There is nothing I know of that does everything you require as simply as iMessage, but I can give you my solution for this (since this is something that I also require).
For quickly sharing links & notes between iOS devices and a Mac (as well as PCs running Windows or Linux), I use simplenote.
You can create a free account and then start using simplenote on all of your devices, simply through their web login (you can also buy a neat simplenote app so that you can store your notes offline, when you are not connected to the internet, but this is optional).
For quickly sharing photos between iOS devices and a Mac, I use instagram.
Finally, for when I don't want to write a note to myself, but just simply take note of a particular web site (or URL), I use instapaper (which is free a web based service that you can use, as well as an optional iOS app that you can buy).
Please note, since instagram and simplenote both have published their web api's, there are a number of instagram and simplenote clients that you can install on your Mac (again, this is optional, since you can just use their free web based service).
The following are optional apps you can install on a Mac :
For the Mac, I use nvALT, which syncs my simplenote notes without needing to use a web browser to login to their web site.
Also, you may find Screenstagram a nice instagram screen saver on the Mac.
For a full fledged instagram client, try instaDesk, or Carousel.
Hope this helps.