Electrum is a popular bitcoin purse service. It is free, actively developing, open source, and supports all popular desktop platforms: Windows, OS X, Linux. There is also a mobile version for Android.
Electrum purse official website: https://electrum.org
Further down we will highlight the main principles of work with the desktop version of this purse, so you could get your collateral from Biterest.
If you use Windows or OS X, you will need to download the purse installation file from the official site and run it on your computer.
The page where you can download an up-to-date version of Electrum: https://electrum.org/#download
If you use Linux, you will need to install the dependent packages:
sudo apt-get install python-qt4 python-pip
After this you can install the purse:
sudo pip2 install https://download.electrum.org/3.1.1/Electrum-3.1.1.tar.gz