<Tab>. The command line now says
If you now type a
rhino) and then
<Tab>twice. The first time around, there'll be a beep, and the second time, there'll be a list of possible completions.
People seem happy about this feature when I show it to them. Myself, I'm even happier because I got to implement the (top secret) mechanism that does this under the hood. I could tell you more, but I would have to kill you.
Update: I forgot to say that this goes for Bioclipse 2 only, but I've now backported the feature to Bioclipse 1 too.
Update2: Tab completion now in the Ruby Console, too.