You should give <a href="https://workflowy.com/" rel="nofollow">https://workflowy.com/</a> a try.
Probably the best thing the internet threw at me last year.
While it's great for todo you can do lots of stuff with this website.
Fantastical integration with Reminders for the longer term stuff. It's lovely to have your todo and calendaring information in one simple app. The natural language processing rocks. Like a little assistant. They have an iPhone app in addition to OS X.<p>For the day to day stuff, good old memory or per-project notebooks.
I'm still looking for one where I can share with specific people (who can then update), and sublists. Closest thing I can find is Workflowy, possibly Cotton, but neither say what each person did, or even what's new.
I'm happy with Remember The Milk (and pay the US$25/year to get it to sync properly with my iphone; my girlfriend is happy using the web version on her ipad), but I haven't tried anything else to compare it to.
todo.txt (<a href="http://todotxt.com/" rel="nofollow">http://todotxt.com/</a>). It works from the command line, has mobile apps, syncs nicely with ssh / dropbox, and saves into a simple to read / port text file.