There are quite a few "sandbox" games available on iOS, including the uber-popular Minecraft, and Majic Jungle's great Blockheads. But my favorite so far is Junk Jack, a 2D sandbox title put together by Pixbits. It's excellent -- its little square tiles are perfect for the iPhone's interface, and almost everything you can craft in the game is useful and interesting. There has been murmuring that Pixbits was going to create a "Junk Jack 2" release soon, but instead, the company has announced a game called Junk Jack X, a brand-new installment of the series with multiplayer, character customization, worlds support, a deeper crafting system and of course upgraded graphics.
You can see what the game looks like in the announcement post, and it's definitely very impressive. Liquids will now flow through the game realistically, you can equip and upgrade various items in a few different slots and the game will also have other planets (!) and themes to explore. Junk Jack is a great game, and Junk Jack X looks like it improves on almost every feature. Pixbits says work is going well, but given the vast scope of the title, we probably won't see it on the App Store until Q3 or Q4 of this year. We'll be waiting.