The downside is that it still does’t have a lot of the features we’re used to from traditional issue trackers (e.g. even its search seems sub-optimal right now … but they’re working on it).

Did you give Trello a shot? Trying playing with it for 10 minutes, you get the feeling of the product pretty fast.

For one collaborative project across continents, I’ve had the chance to use Basecamp (see basecamphq.com), which isn’t free. One of the collaborators got us a basecamp project under the courtesy of his employer. It is quite intuitive, yet leaves much room for customization, and does not force you into a specific pattern of work.

It certainly contains more features about estimating and predicting features size / project velocity. There are a few quirks there that take some time getting used to, but it’s a great tool – I’m not saying Trello >> Pivotal.

I do believe however that Trello is easier to use for people who aren’t familiar with Agile/Scrumm and Poker Planning methods. It took me a while to explain Pivotal to my team at Google, and I could see that the concepts weren’t sinking in with some of them. Pivotal really is more powerful … but if you don’t care about medium-long term planning, and are mainly concerned about what you’ll be doing in the next few weeks – Trello is simple and gets the job done.