I use, recommend and love Basecamp, and now, it is available for mobile devices, how cool is that!  I also need to mention that I liked the fact that in the process of creating Basecamp http://37signals.com/ deeloped the open source web development frameork http://rubyonrails.org/ and wrote the book http://gettingreal.37signals.com/ wich I liked reading as well as http://37signals.com/rework

No apps required. Simply visit basecamphq.com
on your phone’s browser, and you’re good to go!