Twitter mock up is a sample taken from mockupstogo.
One of the downside of Balsamiq is that you need to purchase a license to save your mockup. It also provides free licenses if you fulfill the following criteria -
We are very happy to donate our software to non-profits, charitable organizations and other do-gooders.
If you go to their link http://www.balsamiq.com/customers , you will see who uses their products. There is a huge list companies such as Adobe Systems Inc., Apple Inc., Autodesk Inc., BT Internet, Dropbox, IBM,Microsoft MacBU,Rent.com, eBay, The WorldBank, Time Warner Cable, Yahoo Inc. to name a few.
You can also find other similar software.