[TextWrangler][1] is a great tool for opening editing and saving large files. I wouldn't recommend if for copying and pasting large amounts of content though. Use it if you don't want to have to deal with the vi interface/commands. Like Loic mentioned, MacVim is a great app if you're more familar with vi.

Use [BBEdit for free now that TW is discontinued][2] if needed. 


  [1]: http://www.barebones.com/products/textwrangler/
  [2]: https://apple.stackexchange.com/a/3448/5472