Keyboard-only folder navigation with auto complete
Finder offers the Go to Folder function (found in the Go menu, or by pressing ⇧⌘G), which allows you to type in a path, and has tab-completion (i.e. typing /us TAB r TAB doc TAB gets /Users/rob/Documents/).

It's not perfect, the main downfall is that it doesn't present multiple options, it just chooses the last alphabetical match (i.e. if you type do, in a folder with Documents and Downloads, it will auto-complete as Downloads), but it is a decent built-in functionality.