Vim’s Ctrl+N generally works like this: I type few letters, hit Ctrl+N, and Vim provides me with completions based on words in my all opened buffers.
Solution for Emacs doesn’t have to be identical. I mainly use it like this: declare variable, then use it in later code. But I like the lightweight approach of not parsing the source code.
I personally use AutoComplete It gives you a nice dropdown box. You can select how many letters you want to type before it activates and customise what you want to show up, including stuff in dabbrev-expand.