This post covers public websites with features useful for the game designer during design. In other words, this isn’t about publishing your game (that’s another post).
Often, you’ll need to share game materials with other players, or you want to work on your rules from multiple places. Some folks forget the utility of Google Drive. You can create a shared folder on Google Drive for your game, then drag-and-drop your rules PDF into there and share the folder with playtesters. Or you can write your draft rules in a Google Doc, which you can access from any PC.
If you’re creating a game with components, like a board game or card game, or an RPG that uses special counters, The Game Crafter is invaluable. You can not only buy components (tokens, pawns, miniatures, etc.) in any amount you choose, you can actually build a game: upload art for custom cards or a game board, list the components, and order a copy. Prices are reasonable, and you can buy just one copy of anything (in other words, you never commit to a 10,000-copy print run). Particularly useful for RPG designers who use custom cards.
If you’re writing a tabletop RPG and want to self-publish the rule book, Lulu is one of the go-to self-publishing companies out there. Just upload your book as a Word document (or similar), and they’ll create a book out of it, which folks can buy straight off Lulu; you get the money dropped into a PayPal account once a month. They offer a reasonably wide selection of book types (softcover and hardcover at various sizes), and reasonable prices. Just as with The Game Crafter, you can buy as many copies as you want; you’re not ordering a big print run.
If you know of any others, please let me know in the comments.