Or even better, rbenv: https://github.com/sstephenson/rbenv

Better how? I used rbenv before using chruby and while rbenv is a huge improvement over RVM, I've found chruby to be a further improvement over rbenv.

Since I switched to rbenv I have never needed RVM

Silly man, that wouldn't require $50,000.

RVM is larger and does a whole lot more than chruby. For one, it's much more friendly to new users and doesn't suffer from usability problems like these:

https://github.com/postmodern/chruby/issues/212

https://github.com/postmodern/chruby/issues/213

RVM doesn't suffer from those specific usability problems, but it most certainly does suffer from usability problems. I've wasted many, many hours debugging issues caused by RVM. For the most part, those issues are because RVM is larger and does a whole lot more than chruby, and since switching I haven't once missed any of those extra features.