In addition to those mentioned above there was Dr James Barry, who joined the army in 1813 as a surgeon, served for 40 years, fought a duel, and was only discovered on her death in 1865. John Brown was a soldier in the Royal Africa Company who fell sick on board ship. The surgeon there 'ordered her a glister, which when his mate went to administer, he was surpriz'd to find more sally-ports than he expected.' Dorothy Lawrence served as a sapper in WWI until exposed, while Jane Meace, who probably only joined up for the bounty, lasted until the next morning when one of the men tried a coat on her for the fit. These, of course, are only the ones we know about. In 1644 Charles I banned women who were with the army from wearing men's clothes, but that could simply have been for convenience on their part.
I wrote an article on the songs and some of the real women in Issue 3 of Folk Leads, which should soon be available on line.