On request, the hats I’ve found in books and online worked flat:
*note: For me, I like working in a textured yarn if I’m going to knit a hat flat and sew up a seam. That way, the seam is much less visible.
Books:
Vogue Knitting Caps and Hats (from the Vogue Knitting on the Go series): has lots of hats of all sorts of types, traditional and funky. Many are knit on strait needles. There are some for circular, that only go to dpn for last decreases at the very tops.
Hip to Knit: there are a couple of hats in there, and 2 for strait needles. Also, lots of other basic patterns from classy to functional. They range from very easy to intermediate, but none are that hard, really. Comes with good advice on how to work with any special techniques (dpn, cabling and reading cable charts, etc). Also talks about altering patterns for size.
The knitter’s handy book of Patterns: has a section on hats, and includes info on knitting them flat instead of in the round. Also, generic patterns for all sorts of things in all sorts of sizes, to give you a rough pattern of common items (socks, gloves, sweaters, scarves, mittens) in all sorts of gages and sizes… very handy, just like the title says 
Online:
Down the patterns page, there is instructions for a simple hat knit flat.
This page:
http://www.knittingpatterncentral.com/directory/hats.php
from this site:
http://www.knittingpatterncentral.com/directory.php
has hats on it for both in the round and flat.
I know there are more out there, and I hope people can add links and books to more