In order to use FindDomains you need to create an appid from Bing Developers at this link. It will be like that : 32AFB589D1C8B4FEC73D4BCB6EA0AD810E0FA2C7. When you have registered an appid, enter it to the appid.txt which is in the program directory.
Features
Uses Bing search engine. Works with first 1000 records. Multithreaded on crawling and DNS resolution. Performs DNS resolution for extracted domains to eleminate cached/old records. Has a console interface so it can be very useful with some command-line foo. Works with Mono. But running under Windows is more efficient.