I made a note a while back to try a cross platform app called nvPy, but still haven't gotten around to it. Supposedly, it is a program to replace Notational Velocity which runs on anything by virtue of being written in python. It is still on my to-do list, so I can't say for sure if it lives up to the claim, but you should check it out. If you can't find it by Googling, reply and I will find the link next time I am at a computer.
I have not seen this one yet but I will check it out. If it handles purely in plaintext I may have to switch.
Would something like notational velocity (for osx) and notational acceleration (android) work as a combo well enough for you? the osx app is designed to work with simplenote (iOS only for now) but there are other android apps that may work with it (like NA)
The simplenote clients format to RTF unfortunately.
Have you tried Simplenote? If not through a 1st party app, it is supported through a 3rd party app on almost any platform. You can find the list of all 3rd party apps on their own website, so it won't even be a tough hunt searching for the other clients.
Justnotes is a simplenote client and that as well as Notational Velocity format over to RTF.
It's the clients that are formatting as RTF, not SimpleNote. Perhaps those clients have an option to disable RTF or maybe you could use other clients.
Source (talking about Notational Velocity):
"Furthermore, using separate RTF/TXT/HTML files as a backing store works as you would expect (with full round-tripping of creation and modification times to boot). Conversely you can also keep your notes encrypted on disk, though be forewarned that they'll be sent to Simplenote as cleartext."scrodlog.notational.net/interesting-facts-about-si...
I use one called (oddly enough) PlainText on iOS. Syncs to Dropbox. I just open in Whatever text editor on my Mac or Windows boxes (save in UTF-8 from Notepad).
Not sure how this fits in with Android, sorry.
I'd vote for choosing the best option on each platform and using Dropbox to sync. Otherwise, you'll be locked into something that may be good on one platform and just so-so on another. And no matter what, you'll be using some service for cloud-based syncing, so why not go with Dropbox, which offers you the flexibility to work with any app on any platform?
Dropbox has a plain text editor built in now.