I don't think that we'll be doing MQTT hosting. It's really a core part of what we do not to see your data. However, you have the following options I know about:
-CloudMQTT have a free tier. If you use it, remember that they only support secure websockets, so select that in our 'connection' properties.
-If you are using a Mac, we have created a desktop app that has an MQTT broker included, as well as a serial <-> MQTT bridge for using ThingStudio with plain 'old Arduino's. http://blog.thingstud.io/recipes/how-to-use-a-plain-ol-arduino-with-thingstudio/
-I've also written a blog post about setting up a Raspberry Pi with the latest Mosquitto + Node Red. This is a really sweet combo. http://blog.thingstud.io/recipes/how-to-make-your-raspberry-pi-the-ultimate-iot-hub/
On windows, Mosquitto supply a pre-built binary.
You can also try the open server at http://test.mosquitto.org/
All of which pretty much explains why we are not providing MQTT :-)
Blog posts on : blog.thingstud.io
A wee update blynk.cc went live on App store a few days ago it crashes all the time :( & as @Gordon has said their API sucks (my words not his!) evothings.com this hasn't moved on much since I first discovered this a long while back.
This brings me to @mkarliner thingstud.io thought as Mark say's the UI isn't that sexy at the moment it's the easy is's to integrate too. All the services I've listed he's the only one who's bothered to come to talk to us.
@mkarliner one question for you? have you considered offering MQTT hosting as you've solved one part of the problem but left many with a fence they rather not jump e.g. setting up an MQTT broker and I can't find a free cloud service and HiveMQ is near 90 EUR a year!