NerdTree is a fantastic vim plugin. But for me it was a bit overkill because all I needed was to view directory tree and open files. It also made me feel sad for built-in (and very functional) explorer. Here's a quick addon to my vimscript for easy toggling of the default explorer.
"Toggles explorer buffer
let expl_win_num = bufwinnr(t:expl_buf_num)
if expl_win_num != -1
let cur_win_nr = winnr()
exec expl_win_num . 'wincmd w'
exec cur_win_nr . 'wincmd w'
exec '1wincmd w'
let t:expl_buf_num = bufnr("%")
"Set default width of explorer to make it appear like a sidebar. Also defaults to tree style.
"Lastly, set a key mapping for calling the function above
noremap <silent> <C-E> :call ToggleVExplorer()<CR>
I picked it up off net a long time ago and have really forgot where I got it from. Kudos to whoever wrote it :)
PS: I use ctrlp.vim primarily for quickly opening files and managing buffers. Sometimes, you just have that itch to see the directory tree though.