I've been taking a look at the public beta of the next version of PL/SQL Developer. I'm currently using the 8.0 release, choosing not to upgrade to 9.0 when it was released last year. One of the new features Allround Automations added in 9.0 was the ability for one instance of the application to open sessions to multiple databases. I considered this a pretty risky enhancement.
However, in the upcoming 10.0 release, this multi-session feature seems to be refined and more solid. One nice enhancement is the ability to assign a window background color to specific connections.
Here, I am connecting to the QA, UAT, and PROD environments:
Now there is much less chance of accidently running a data update on the wrong DB! This feature would have been quite useful in 8.0 and earlier, frankly. It's nice to have an obvious reminder of which environment you're looking at.