There are lots of good apps / services out there. Cross platform, I think that Trello and Todoist are great.
If you’re on the Apple platform and a fan of Getting Things Done, OmniFocus is a great tool. It is a little pricey.
And with v3 of OmniFocus there will be a web-based view which will allow so collaboration- although this is going to cost a little more I believe.
I was always happy with Wunderlist but jumped ship when it was announced it would be closing down, so for probably a year now I’ve been using Any.Do which I find works really well. It definitely helps me remember and get stuff done. 