Smultron's implementation of projects isn't as smooth as TM, but yes it can open a whole directory. However, it doesn't preserve the file structure, it just gives you a huge listed (not nested or anything), and it can take a long time to open a large number of files. I haven't played with TW, but BBEdit–sort of the pro version of TW–has a pretty decent implementation of handling projects/folders.
I used to use Smultron exclusively, it works well for most things, and it's my recommendation for people looking for a solid free editor.