I have a few stories, but the one I'm going to focus on is the hardest breakup I ever went through, which happened in February 2008.
Long story short, we were in the midst of preparing to move to Seattle from the midwest. I went to Seattle to search for apartments, he stayed behind. I was in Seattle for a week (Wednesday to Wednesday). I couldn't get a hold of him Saturday and most of Sunday, for which he stated he worked a double shift (found out later that the he didn't show up for work at ALL the entire weekend) and that he left his phone at work, and he didn't get it back until Sunday afternoon.
Two weeks later, I get our cell phone bill and I see that it is $70 more than it normally was, all due to MMS (picture text messaging). Luckily, with my phone provider, you can go online and see the pictures that are sent via MMS. I logged in and found a TON of pictures he sent of himself in various states of undress, including full on nude pics, and countless pictures of his cock, in various states of erectness. Through the text that went with the pics, I found out that the guy he was sending these to had fucked him in OUR bed, in OUR apartment (all of which I paid for, mind you). I contacted the guy and got the real story. I found out that my ex had told the guy that it was HIS apartment, and that we had broken up and I moved to Seattle. The guy told me he thought it was awfully strange that he was a barista at a coffee shop, but could afford an apartment like the one we had, in the area of the city where it was (it was in the upper crust area of the city). At that point, I loaded everything he had into boxes and stacked it by the door. I printed out the pictures and text messages, then went and picked him up at work. We got home, I confronted him with all of the information I had accrued over the previous 3 hours. He denied ever having sex with the guy, but I had proof that he did. I immediately threw him out, onto the street, and he had NOWHERE to go. I spent the next three days and nights in a drunken stupor. It took me over a year to get over that.
If you feel the need to cheat, then cheat. But DON'T cheat in the bed that belongs to you and your boyfriend/girlfriend. That's just adds a whole new level of wrongness to an already fucked up situation.