Leslie asked what patterns I use for my nightgowns, and I had to stop and think a bit before I could answer. For over a decade, I alternated between the two nursing gown patterns (#104 and #105) from Elizabeth Lee Designs; both of which I highly recommend if. . .well, if you need a nursing nightgown. 
It’s only been in the past year or so that I’ve needed to find normal patterns, although as I recall I made a couple of nightgowns B.C.
So. After giving it some thought, I realized I can answer the question with two words: Kwik Sew. 
I’ve purchased a couple single patterns, but mostly I rely on their sewing books. Beautiful Lingerie offers basic ladies’ gown patterns with plenty of variations and ideas for trim. For Twirly Girl, I have used Sewing for Baby, Sewing for Toddlers, and now Sewing for Children. All three books have basic patterns that can easily be adapted to make nightgowns or nightshirts (as well as pajama tops & bottoms, of course). Again, there is plenty of room for variations and trim to make personalized garments.
Ottobre has also offered a couple of pretty nightgowns over the years. Twirly Girl has had more than one gown from design #10, 4/2003. I haven’t sewn her #18 from 4/2005 yet, but I have sized it up to make a gown for myself. My most recent gown is actually a modification of dress #1/tunic #3 from 5/2008: I omitted the belt and center back seam (having realized I can put my blouse on without the zipper) and added enough length to reach my ankles. I also changed the tucks to gathers to speed the construction process.
I’m sure the major pattern companies also offer nightgowns, but the closest fabric store is over 2 hours away so I usually stick with patterns I already have at home. For something like a nightgown, I find it easier to make minor style modifications to a pattern that fits than to start with an unknown fit.
