I have come back to this and am stunned at how many of the posts here resonate with me 100%. Some of the stories are horrific 
. I have long wanted to start a thread like this but having done my story to death on the Relationships board, I didn't want to be a complete bore-fest. Thank you OP for starting it.
My now ex-h left me in October 13 after 14 years together. I had one DD and together we had one DS who was 2 1/2 at the time. It took me a little while to find out about OW. She was somebody known to us, we first met her in 2002. Unbeknown to me, my ex-h had been in touch with her throughout our marriage. She was also married and had one DS. I subsequently found out that her husband had been killed in March 13. I believe they were having an affair at that time. My ex-h says they "bumped into eachother" only two weeks before he left during which time he cancelled all our utilities, cleared out the bank account, re-registered his business at hers, and put himself on her council tax. He sacked me from our company without notice or pay and left me on benefits. As you do. Within a couple of weeks I received a petition for divorce on the grounds of my "unreasonable behaviour" 
that was so full of shit I confronted my ex-h to which he said "I had to say something". Basically, he made it up. He had not told his solicitor he was having an affair. He told me that the divorce had to be my fault, that OW couldn't have any "slight on her business and reputation" and had enough money to "wipe me out" if I basically didn't do as they said. I shan't bore you with the quite astonishing email I received from her. I didn't accept their version of events and thus the shit hit the fan...
It was all my fault. I was cold, frigid (except when he was shagging me during the week before he left obviously), I had no emotion, I was a shit wife, a shit mother, he stayed to protect the children from me, I had never made any financial contribution to the marriage (of course not, worked and paid the mortgage throughout). If it wasn't for OW, he would be a "broken man". What an utter wanker. He KNEW what he was saying was bollocks, but he had to do it to cover up their shitty, vile behaviour. Then she started....he assaulted me in front of our DS and was arrested. She wrote me the vilest email telling me I was a "nasty, horrible no mark", "if you were so great, why did he leave", "something off of Jeremy Kyle, two kids by two different fathers", "she was bored my pitiful bleating", I was "too vindictive to see reason". Oh it went on and on and on. Prior to that I had had some other shit, nasty emails from her. She is malice personified. I managed to get an adultery petition to the court and they exploded. It went from bad to worse. I could go on and on and on. The funny thing is, OW is much older, hideously unattractive and is terribly terribly insecure. She has turned my ex-h into a fat, ugly, lined, yellow lump, cut his hair into a ridiculous hairstyle, his clothes are out of date. She is so frightened she is going to lose her "prize" that she has made sure that nobody would even look at him. He looks like a bloody broken man now!
She has driven such a wedge between us that we can no longer communicate. Her finest hour was when she decided my ASD son was not ASD as she was a "children's hairdresser with a CRB check" and I was "attention seeking and doing it to claim DLA". Thus, ex-h checked out the process completely and offers no support to our son whatsoever. My ex-h has acknowledged on the very odd occasion that he is a complete shit. However, he "had" to do it. He even, on one occasion blamed our DS saying that he had been forced to "do this" to ensure he grew up with "love and emotion". What the actual fuck? The damage those two have done to my kids is incalculable. I know they are widely judged, particularly due to OW's dead husband. However, they are perfectly justified as I was such an awful person and my ex-h told everybody I had "mental health issues" and he knew he had made a mistake two years after our marriage! What a guy, staying for all those years and then leaving me with the baby he pressured and begged me for.
Sorry, this is a bit stilted and long...but you get the picture. Their behaviour has ensured that 2.5 years on, I am still struggling to recover. I hope they rot in hell frankly. They deserve eachother. to each and every one of you going through this hell.