I've used BaseCamp extensively and like a lot. Mobile site is awesome. I've mostly used for myself, to track tasks and projects and record progress and issues.
Because these tools use Email to send updates, you don't need to use the website. You can simply use your mail client to respond to messages from others, but the website records and chronologically orders all responses. And you get notified when there is a deadline or milestone approaching. Because I do Management by Priority, this usually adjust my focus and improve concentration. :)
I'd love to hear if anyone has good or bad experiences with one or another tool and why.