Ruby and Rails in 2004. I'd been a Perl developer for 8 years till that point but had basically given up on building anything substantial with it and was transitioning to being a freelance writer. Ruby and Rails got me straight back onto the development track and had a huge influence over my successes since.
2006 for me after 9 years of Java and a few of C, C++, and a smattering of PERL and Python. Rails, and then Ruby, were such a breath of fresh air after the verbose hell of Java, J2EE, and Spring.
