Web/iOS/Android/Chrome: If Astrid's pending shutdown has you bummed, or you just want a syncing to-do app with mobile apps, solid recurring task options, and a few collaboration tools, TickTick is a new kid on the block that's worth a look.
TickTick was built by the same folks behind GTasks. It's in beta, and the web interface is pretty sparse, but if you loved Astrid, you'll be familiar with the look and feel of TickTick. Plus, you get seamless syncing between the webapp and its iOS app (the dev says an iPad optimized version is on the way) and Android apps, and a Chrome extension if you'd prefer that to a pinned tab.
Adding new to-dos is as easy as typing them, and you can set deadlines and set custom reminders for specific times of day. You can set the to-do to recur every day, week, month, or year, complete with specific repeat days, so you can have a daily to-do repeat every five days, for example. You can even set when the to-do should stop recurring. Like any good to-do app, you can organize your items into lists to keep them separate, and get a daily or weekly view of everything you have coming up. You can add notes to individual to-dos, create lists with to-dos and subtasks beneath them, and share to-dos with other users.
Aside from that, there isn't too much else. The developer has "labs" features you can enable, like search, or a calendar view, but some of them are limited to TickTick's $10/yr "pro" accounts, which don't really get you that much right now, but as the service grows, it can either be a great way to try out new features or a way to nickel and dime people into upgrading. You can check out the plans here and read more about its features here.