Post Prohibition: A Baltimore-based movement celebrating the craft of the cocktail. In addition to cocktail recipes, you will find an entire DIY section on liqueurs, bitters, and syrups.
Serious Eats: I love their “DIY vs. Buy” series, which has covered everything from celery bitters to banana liqueur.
12 Bottle Bar: If you’ve ever been frustrated by cocktail recipes that call for half an ounce of some obscure spirit you may never use again, you’ll find the rigorous minimalism of 12 Bottle Bar most refreshing. There are a wealth of cocktail recipes on this site, but none of them stray beyond the carefully curated 12 bottles.
Jeffrey Morgenthaler: Although it hasn’t been updated in a while, there’s a treasure trove of tips, techniques, and recipes at Jeffrey Morgenthaler’s website (he’s the bar manager at Clyde Common in Portland, Oregon).
Home Distillers: There’s a section on liqueurs that collates a lot of information on possible ways to achieve certain results, such as Frangelico, Goldwasser, or Kahlua.
The Joy of Mead: A great crash course on mead-making, plus lots of links for supplies, books, and other mead-related sites.