The original Spelunky - the free, low resolution Game Maker-made Classic version - is open source. That means it can be broadly modified, which is what Ukrainian programmer Vadim has done in Spelunky SD. The project merges features from the polished remake Spelunky HD with the lo-fi original, most notably adding two-player, online cooperative multiplayer.
Local co-op was one of Spelunky HD's main features, letting players descend into the game's procedural dungeons with a friend along to provide
help an extra threat of death. Spelunky SD technically goes one better, by enabling online co-op so people can play over the internet.
As explained by a post on Vadim's site, the addition was harder than the programmer originally realised:
As I have already mentioned, the original game is single-player. There's only one instance of player, and game does it's best at working with it. So, what happens when you add another player? Nothing considers it. Enemies will not chase the second player, traps will not trigger, and overall it will seem that second player is merely a ghost or a product of imagination.
Another thing that I did not consider was migration bugs. Migrating source over two major versions of program is already no easy task, and, while I wouldn't say that game is poorly written (neither it is possible to foresee all possible changes to development tool), after being migrated over to GameMaker: Studio, in some cases code would behave slightly unexpectedly at best.
Aside from co-op, the mod adds other tweaks present in the Spelunky HD remake, such as sound effects fading when off-screen and enemies blinking on and off to indicate when they're about to get back up. Even if you never intend to play online, quality-of-life additions like those make returning to Spelunky Classic more appealing.
Spelunky is, by the by, one of the best games ever made. It's probably my favourite. I reviewed it in another castle.