I believe vim has one built-in called OmniComplete. In insert mode use
Otherwise you can do keyword completion from insert mode with
ctrl+n. When you invoke it it will give you a dropdown displaying possible matches of words found in the file you're editing, as well as found from other files you have open. Keep hitting
ctrl+n to cycle through the options. When you come to the word you want, just continue typing like normal.
If you can't find the word you want,
ctrl+n will eventually cycle off the list and you can keep typing, or you can hit
ESC and then
bcw to go
back to the start of the offending word and
word to the one you wanted.