Vim-autocomplpop is a fairly old plugin, but it still works just fine. It automatically shows autocompletion options as you type like an IDE. This is very handy.
The only downside is that external autocompletion engines like ctags and rope can sometimes cause performance issues. For example, python.vim uses rope to analyze a file, but that can take a while to initialize and crawl directories if you're editing a file in your home directory. Since it's a plugin written before Vim/Neovim's async code, it tends to make Vim unresponsive, so I only enable vim-autocomplpop on projects I'm working on. This isn't an issue if you use Vim's built-in autocomplete (instead of python.vim). I'll get around to rewriting autocomplpop someday and working on a solution to mitigate external autocomplete providers slowing down Vim.
0: https://github.com/othree/vim-autocomplpop 1: https://github.com/racer-rust/vim-racer