The wonderful Stewart Sharpless scoured the plates of the Palomar Sky Survey and published the Sharpless Catalogue of H II regions in 1959. It contains 313 entries. Whilst the list contains some very familiar targets such as the Orion region, there are a wealth of faint jewels for the narrowband imager to try his luck on. There is something for any image scale, and I particularly enjoy imaging the fainter members of the catalogue which boasts some very delightful objects that do not get the coverage they deserve.
The starting point for any investigation into the Sharpless catalgoue must start at the The Best of the Sharpless Catalogue website which helps you sort out which ones fit your FOV and equipment. Be warned, most of the faint stuff needs super long integration times with a narrowband filter. Most of the hydrogen alpha images on this page start at around 4 hours exposure time, and some have near 12 hours.
I also have my own list with images of the complete Sharpless Catalogue