Hope no one minds a new post here--the others looked older and I did want to chime in.
I seem to be a cookbook collector. I just can't help buying them off of the bargain rack at Borders and I've got a whole bookshelf full at this point. Still, there are a few I turn to time and again:
Moosewood Restaurant Cooks at Home
Moosewood Restaurant Daily Special
Joy of Cooking
Fine Cooking magazine
I really like _Moosewood Restaurant Cooks at Home_. It's full of creative, delicious, mostly vegetarian, mostly healthy recipes that I find myself making again and again. Their Pasta Fresca is a summertime favorite of mine, and the Red, Gold, Black, and Green Chili is good in the fall. I make their (very simple) recipe for Easy Elegant Asparagus just about every time I buy asparagus now. So good! I gave a copy to my sister-in-law for Christmas and she's busily trying all the recipes I haven't. 
She gave me _Moosewood Restaurant Daily Special_, and though I'm still exploring it, I like it a lot. It's a cookbook full of soups, salads, and breads to mix and match (with suggestions on which go together--always nice). Already I've made their Cauliflower, Cheese, and Tomato Soup, which delighted my picky husband, and the Wilted Spinach and Portobello Mushrooms salad, which was very good (though best when first made). Not all of the Moosewood cookbooks are great (I have the _Low-Fat_ one, too, and have a hard time finding something I like in there), but these are two I reach for a lot.
_Joy of Cooking_ is a classic that I mostly use as a resource. I turn to it when I need to know how long to cook a piece of meat, how to make a beef stew, what temperature a cut of meat is done at, or any time I need help with a technique or want a basic recipe. It's got just about everything.
I really like _Fine Cooking_ magazine, too. It's worth keeping the back issues on my shelf, both for the recipes and the tips and techniques. Some of the recipes are involved, but the results are often worth it when I want to put in a little time and make something a little fancier. Most are accompanied by articles with tips from the cook that would be cumbersome in the recipes themselves but are helpful in making them or working with the main ingredient.
I also like the Southern Living recipe collections and most Weight Watchers cookbooks. I've got a couple of each and turn to them often. Good stuff!