Hi Keepaway,
Firstly, you are awesome, and you are right to furious and not ashamed. This man is disgusting beyond description.
Secondly, please please please do reconsider about telling his wife. Maybe she does know, but if she doesn't? That's just way too sad. People are saying that she probably knows, but I guarantee that there were people saying exactly the same thing about you when you were together. "Oh she probably knows deep down that he's still married, and is just happy to be the mistress". If you were duped, then the chances are she was too.
This woman is being absolutely humiliated and you have the opportunity to open her eyes. You can't guarantee what she will do, and that's not for you to worry about. But you can at least make her aware, and then put your conscience to rest.
My advice? Print out the e-mails from right at the beginning of the relationship which talk about how he is divorced with an amicable split, and inviting you to dates. Basically anything which shows that he lied about his marital status. Hopefully they are from his work account, so she will recognise that it is irrefutably him. Put them in an envelope.
Print out one or two e-mails where you guys had arranged to do something together on a weekend. Guarantee that she will remember those as dates that he was "working". More lies. Put them in an envelope.
Next print out the e-mails (again from his work account) from more recently after the split saying how sorry he is, how he wants your forgiveness, asking to let you explain. INCLUDING THE MOST RECENT ONE. Then put them in an envelope. You can also do an envelope with pictures but only if you are comfortable with it.
Next, write a very short letter and put it in an envelope. All you have to say is:
I am so sorry. I had no idea. I hope that the following can help you to believe me. I want nothing to do with this person - they are the worst type of scum. I was so scared to say anything, but I couldn't face my conscience by not doing so. I am sorry. Whatever you do from here on is of course only for you to decide.
On your letter write "for Mrs X- first"
Then on each of the others write 1, 2, 3 etc. If you do a photo one, please write on the back "this contains photographic evidence" so she is forewarned.
All you need to do is drive to her house when you know that he will be out. Ring the doorbell. When she answers, asks if she is Mrs X. When she says yes hand her the envelopes and just say "I'm sorry" then leave SHARPISH.
This way she has irrefutable evidence against her husband if he tries to deny it. She can either confront him, or burn it and never mention it again. You don't need to know what she does.
I find it so sad when people find out that their partners are really married, but never say anything. It's basically enabling the guy to get away with it.