I would make kimchi. Here's a quick vegetarian version:
http://jeanetteshealthyliving.com/2011/06/quick-vegetarian-napa-cabbage-kim-chi-kimchee-recipe.html

Or you can make the non-vegetarian version:
http://www.girlcooksworld.com/2011/04/napa-cabbage-kimchi.html

Cabbage rolls! If you're not into the traditional red meat filling, could always make them with ground chicken or turkey instead. And they freeze like a dream.

You can steam it and use it to make korean veggie wraps. Take a leave and throw in some rice, kimchi and whatever meat you have, roll it up and enjoy!

Kimchi!

I like making a simple broth based soup with tofu, napa cabbage, rehydrated shiitake mushrooms, and broth of choice. For a more umami blast, I like adding dried scallops into the mix. I like the sweetness the napa cabbage imparts to the broth.

Pork and cabbage potstickers or dumplings would be tasty as well.

I just made cabbage and grapefruit salad this weekend, and while I'd recommend it, I'd also say to use about half as much dressing, and only dress it about a half hour before eating it. Two-day leftovers weren't horrific, but they weren't anywhere near as great as the salad to start with.

Stuffed Cabbage... my favorite.
http://esseppis.blogspot.com/2010/06/cabbage-cupcake.html

Napa cabbage loves soup! I like to make a chicken broth based udon noodle soup, add copious amounts of ginger and napa cabbage.

I second kimchi! I just taught a kimchi-making class here in Philly and my students were suitably impressed with the ease of turning a humble head of cabbage into a delicious meal/condiment/gift from the heavens.

As an alternative, what about fried rice (or almost fried). I make brown rice, topped with whatever frozen veggies I have on hand (usually peas), then do a quick sautee of onions, carrots, garlic and nappa cabbage, mix in a couple eggs. Once the eggs are cooked, combine with rice. Top with soy sauce to taste (and in our house some kimchi). Quick, filling and tasty!

You MUST make this Vietnamese Grilled Steak and Napa Cabbage Salad from Melissa Clark's cookbook "Cook This Now". It's to die for. Sub tofu for steak if you're veg.

http://www.stltoday.com/lifestyles/food-and-cooking/recipes/vietnamese-grilled-steak-and-cabbage-salad/article_098e7442-d4c7-5c26-af25-0cdcebfd66d5.html

How funny, I'm currently working my way through a massive head of napa, too! I bought it to add to some pad thai (I know, not standard, but I wanted something crispy and shredded w/the bean sprouts), then I started working my way around SE Asia and managed to find plenty of dishes to throw some in (pho, hot & sour soup, pork/cabbage spring rolls, random stir fries, curries, etc.) and now I've made some fresh slaw for some bbq I made, and I always make my "unstuffed cabbage" when I have the dregs of a head left - it's all the same ingredients in a standard stuffed cabbage roll recipe, except I roughly chop the cabbage and just let everything simmer together in a pot. It's so much easier than rolling/stuffing and it tastes just as great.

I buy it to put in with noodles in broth and lots of garlic, soy sauce, hot chili oil etc. I am so happy it is cold here again because I'm ready to tuck into some of that.

I love this salad, it's more of a summertime thing, but you can easily get through a head of the stuff: http://www.epicurious.com/recipes/food/views/Napa-Cabbage-Tomato-and-Avocado-Salad-236677

Dumplings! http://the-cooking-of-joy.blogspot.com/2008/12/another-favorite-thing-my-mom-makes-are.html

And you can use napa in hot pot too. http://the-cooking-of-joy.blogspot.com/2008/12/one-of-few-holiday-traditions-my-family.html

I second the above comment to make dumplings. Also, stir fry.

One of my go-to recipes for leftover cabbage of any sort is Epicurious' Mu Shu in moments. It's great both freshly cooked and leftover.

http://www.epicurious.com/recipes/food/views/Mu-Shu-in-Moments-233797

I was also going to suggest kimchi and/or golumpki. I make golumpki with ground turkey and they are so good.

I make pad thai coleslaw, and wrap in collard greens. Healthy and delicious.

I use Mark Bittman's peanut sauce, which is a little spicy but divine as a lunch wrap!

I'd make hakusai tsukemono, use it in gyoza or egg roll filling, or stir fry.

This is one of my favorite recipes - quick to make and the kind of thing I enjoy on a cool fall evening: http://centerstagewellness.com/hot-and-sour-napa-cabbage/.

I just cut it up into squares and then melt some butter, fry it until it's soft, add some salt and pepper and call it a day. Very easy and very delicious.

Make sukiyaki!!!
http://www.youtube.com/watch?v=eTCiQdoWYgU
On the cheap, make ramen soup. Add seasoning packet and soy sauce to water, boil. Cook whatever you like in the boil (I like edamame or a frozen Asian blend), then cook noodles. A little before the noodles are done, add in some shredded napa. Continue cooking until napa and noodles are cooked through.
Napa is so delicious!

Similar to Nocturnal's suggestion: I cut into squares. Heat a wok or skillet on high, add veg oil and some smashed garlic. Add cabbage and stir fry til softened, season with salt. You can mix it up however you like- sesame oil, sesame seeds, chilis, spice... Instant, yummy, healthy perfect side for any meat, especially Asian.

Delicious in stir fries, soups, or with fried noodles/rice.

When I am feeling lazy I like to slice thinly and boil with noodles, tofu, and shrimp, crack an egg in, stir in miso at the end, to make a quick, delicious, and healthy meal. You can also stir in leftover rice instead of the noodles. It's a great substitute for instant ramen - much healthier cause you are cooking from scratch and almost as easy to cook.

Thank you all for your help!! I quartered it and I'm roasting one, making one into vegg-o kimchi, one into Elissa-D's salad w/ tofu, and the last is being saved as an addition to my weekend pho. These are some great ideas!!

Is it too late to make more suggestions? I'm chinese, and chinese people loooove napa...
http://www.epicurious.com/recipes/food/views/Lions-Head-Casserole-238436 <--can definitely omit the water chestnuts...the key ingredients are the rice wine, ginger, scallions, soy sauce and some cornstarch for the meatballs. I usually make mine with just those (and maybe some white pepper)...Layer slices of napa cabbage underneath the meatballs after you've semi seared hte meatballs, and cover to cook. the napa cabbage soaks up the juices..oh , and add some shiitake mushrooms, too!

Napa cabbage + some rehydrated dried shrimp + shiitake mushrooms stir fry = simple and fast.

Is it too late to make more suggestions? I'm chinese, and chinese people loooove napa...
http://www.epicurious.com/recipes/food/views/Lions-Head-Casserole-238436 <--can definitely omit the water chestnuts...the key ingredients are the rice wine, ginger, scallions, soy sauce and some cornstarch for the meatballs. I usually make mine with just those (and maybe some white pepper)...Layer slices of napa cabbage underneath the meatballs after you've semi seared hte meatballs, and cover to cook. the napa cabbage soaks up the juices..oh , and add some shiitake mushrooms, too!

Napa cabbage + some rehydrated dried shrimp + shiitake mushrooms stir fry = simple and fast.

This is a great recipe too! Faith's Asian Cabbage Rolls with Spicy Pork http://www.thekitchn.com/recipe-asian-cabbage-rolls-wit-137820

It makes a great cold salad- use it in place of regular cabbage in any slaw recipe and it will eat more like a lettuce. Otherwise- chop it up and toss into stir fries, noodle dishes, or fried rice.

I also like it in salads or slaws...Smitten Kitchen has a few yummy recipes.

This is a rather plain recipe but it's easy to throw together and lends itself well to interpretation. I always use more chili flakes than it calls for:

http://projects.washingtonpost.com/recipes/2009/04/29/shredded-pork-noodles-and-cabbage/