[toolslive]

- IBM's visual age for java provided most of this for Java too. (yes, it was modeled to the smalltalk env with the same name)

- Zope (python app server) had a feature where you could debug a web application (in production) in a private session, so all code modifications you did in your session were private until you committed the code.

[javajosh]

- Jrebel is a commercial tool that also lets you replace bytecode in a running program without loss of state.

[bzzzt]

"Hot code replacement" is a Java VM feature. What makes JRebel useful is support for live reconfiguration of a gazillion Java frameworks (e.g. Hibernate, CDI, Spring etc) so you don't have to restart your server after changing some configuration annotations. It's expensive though and slows down startup so it's a tradeoff.