Well, I'm not sure about all eclipse plugin flavors out there, but in PyDev has an option to "request completion on all letter chars and '_'".
The user may disable it in the preferences, but it's turned on by default...
Now, the code-completion on PyDev is a bit different from most other plugins... usually that completion will give you the simpler keywords initially (like 'self', 'def', etc) and the other completions (which expand based on context or global tokens) starts when the user keeps typing and those are exhausted (or when the user explicitly presses Ctrl+Space).
All in all, I quite like it ;)
Note that in JDT you can mimick that by setting the auto-activation chars as all letters... I.e.:
Go to Preferences -> Java -> Editor -> Content Assist -> Auto Activation and set Auto Activation triggers for Java: .abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ_.