I usually try a@b.com for forms I don't care about, but often have to resort to the more verbose nobody@nowhere.com when the validator complains about the former being too short...
When I need an email address for something that should not need my email address and for which I have no interest whatsoever in receiving any emails sent to, I use something @mouse-potato.com.
Mouse-potato.com was registered in 1997 by, if I remember correctly, the owner of the Seattle ISP Northwest Nexus. Northwest Nexus is long gone, but he or someone has kept mouse-potato.com alive ever since.
The MX record for mouse-potato.com points to mail.mouse-potato.com. The A record for that, www.mouse-potato.com, and mouse-potato.com all give 127.0.0.1.
Give some spammer an @mouse-potato.com address, and hopefully they send the spam from the same machine their own SMTP server is on, and so end up just trying to send it to themselves.
After he registered it, he stated publicly that it was specifically registered for Northwest Nexus customers, and anyone else who wanted to use it, for making fake email addresses.
It's just my own go-to (not-) fake e-mail. I probably tried it the first time when I was 12, and it worked, so whoever actually owns nobody@nowhere.com must be getting a lot of junk... At some point I realized the e-mail actually exists, but oh well... I figure with an e-mail like that they never expected it to be exactly private
I own a domain that is similar (in a "1337" way) to the domain of a CRM SaaS operation. At least once a week one of their customers signs up using my domain in a throwaway fake email address. I find it extremely frustrating. Because of their architecture and mine there is no easy way for me to filter these messages so I am often stuck with the spam in my inbox. I don't hold a grudge or anything, but I wish people would think before they fill in someone else's perfectly valid info into a form they don't want to give their own info to.