It would take a lot of effort to knit an American flag (which isn't to say I won't try it some day!), but it's pretty easy to stitch something that looks like a flag without quite so many details.
I really like this American Flag Placemat, which has the proper 13 stripes but doesn't include the stars (mostly because it's worked in Garter Stitch, but you could make the blue section Stockinette and add white stitches where the starts ought to go if you want, or stitch them on after).
There's also a little American Flag Pin, which is in no way accurate other than the colors, but it also takes just a few minutes to knit, which is always kind of fun. It's the perfect accessory for Fourth of July parade viewing.