Celoxis is both an online and installable project management platform integreted with Microsoft Outlook, iCal, Google Calendar, and Salesforce.com, among others. Celoxis explains the Celoxis API in a way that suggests they want you to be able to use it easily. Check out one graphic, by way of example:
Available to both customers using the installed software and web-hosted customers, the API enables you to integrate your own software such as ERP. Since the API is web based and platform independent, programs can be written in the language of your choice, such as PHP, Java and .NET. The API supports both JSON and XML data interchange formats.
The problem with project management software is that given how different the projects are, the software doesn't quite manage things the way you want it to. But according to a customer testimonial, from Orest Werbowy, an Implementation Specialist at Riva Modeling, Celoxis might have cracked this,
"Celoxis has always been very prompt with responding to our concerns, issues and requests. We are also happy to report that many of our bug reports or enhancement suggestions have come to fruition in consecutive releases."
Among other tools, Celoxis uses an interativee Gantt chart for Creating tasks/subtasks, Scheduling, Setting Dependencies and Constraints, Resource Management and Progress Updates
Pricing starts at $14.95 per month per user with 2 GB storage free, $10 per 10 GB after that.