Here are a handful of links to some of the sites I've found useful in years gone by. I'm thankful to all of them for helping me learn more about my hobby.
EasyMAMECab's Modeline Howto - if you've ever wanted to know how a CRT works, this is the guide for you. Sadly the moving images were made in Flash, but you can still enable them on modern browsers like Chrome via a configuration exception. Brilliant information for general understandings of how CRTs work, and how the modelines we generate to play low res games control what we see on screen.
NFG / GamesX Wiki - one of the earlier places that collected pinouts for things and stored them in one useful place. These links alone have asved me thousands in unecessary converters, when making my own cables and hookups were many times cheaper.
uCON64 - the ultimate ROM dumping/patching/checking/merging tool. I started using this on MS-DOS in a pre-Internet era with my Super Game Doctor, and later Super Wild Card DX. It's since continued to grow and support a number of new flash carts and devices. If command line tools scare you (and they shouldn't), uf-FOX is available as a GUI wrapper for uCON64.