It depends what you want from a system.

Is it resource allocation? Task tracking? Client feedback? Time tracking?

If you're looking for a collection of smaller tools, I've used a combination Harvest / Harvest Forecast and Team Gantt. All really good focused tools.

Teamwork provides the most fully featured offering we found but is a bit heavy as a result.

Basecamp is really popular but I've always find it too simplistic.

Asana is another all in one solution which recently got a redesign - not used it though.