Yes, it has been asked (and answered!) at least a miilion times here on AnswerBank! But I appreciate the search function on here can be a bit "hit and miss". The wiki entry is quite accurate. Anyway - here's some of my posts from previous threads;
"If you Google this you wil find out that current white suited Stig is {usually} a racing driver called Ben Collins. He has competed in F3 and NASCAR among other things. The previous black suited Stig was ex Le-Mans driver Perry McCarthy but was "let go" for revealing the fact he was The Stig.
Ben isn't always The Stig though, sometimes the manufacturer doesn't let him do the job and they dress up their own guy (like Luca Badoa for Ferrari) instead"
and
"Believe me or don't, the choice is yours. Personally I couldn't care less.
My brother-in-law is a LCM (lighting camerman) down at Dunsfold for the filming of TG. 70-75% of the time it is Ben, the rest of the time it's the manufacturers test driver (like Luca Badoa for Ferrari) or sometimes Darren Turner if Ben is away racing. Ben was quoted in the H&S report concerning Hamster's crash as well."
Sometimes you will actually see the various test drivers used (like Luca) if they are racing each other (say a Ferrari vs a Porsche) and Stig can only actually drive 1 car at a time! Ex-Renault F1 test driver (2003) Alan McNish has been used as well in the Renault F1 cars and other hot Renaults.