Trello is a free-form tool for making lists. The lists can be shared (for reading and writing). You make as many as you want. When this gets unwieldy, you can start a new "board" - a board is a collection of lists.
If you want, you can add notes to any item in a list, and other people can add comments. There are options for watching items, lists, and boards such that you can get alerts when something changes or is added.
It's very flexible. Spolsky has said the original idea traces back to when he was a program manager at Microsoft on Excel, and during customer visits he discovered the vast majority of users simply used Excel to lay out lists -- they didn't care about advanced features like formulae or macros.
Actually I'd say (and I think Joel, the inventor, said the same thing) that it's a meta tool which is much more versatile than a bug tracker. It's like Excel - people are starting to use it in ways nobody has ever imagined it. I admit that I never imagined it to be used as a household management tool. Of course, one can do Kanban on it, too. Oh that reminds me I have to check out their API, I'm quite sure it's going to morph into some kind of authoritative meta list store.