I saw an article about
Ninja SMS over at Tested so I downloaded it to try it out. It's a fantastic idea, but isn't really ready for daily use yet. I wouldn't recommend buying it right now unless you want to lock in the lower price (Currently 65p for the first 5000 customers).
It's pretty much Facebook's Chat Heads for texts, with pop up conversations, transparency toggling, themes, customisable notifications and the ability to minimise conversations to just pictures which can float on top of other apps and update when you get a new message.
You can also customise which contacts get pop ups and which don't, or just select everyone.
The thing is, it's pretty unstable at the moment. Tested mentioned that minimising a conversation to the picture only can sometimes cause the app to become invisible - this happened to me when testing it just now - the app disappears but still acts as if you're inside text entry - keyboard popups, etc. It also made my phone unresponsive, the home screens wouldn't scroll smoothly until I restarted the phone.
It's now disabled and I've gone back to SMS Popup, but I've not uninstalled it as with some stability improvements it could be brilliant. I just hope it gets an update soon.