I work as a consultant in a medium-sized firm. We need to keep track of time spent on individual tasks and which customers to bill to. Our current system for doing this is an old in-house system, that needs to be retired for various reasons.

Most developers don't like registering time, so I am looking for the best tool for this job, that minimizes the time needed to register time. Must have features:

• Must be simple and easy to use.
• Must have reporting feature to use for billing.
• Must have an API, so we can integrate in-house tools.
• Registering time should be based on hours worked (ie. today, worked on Customer A, Task B from 8 AM to 12 AM).
• TFS integration is a plus, but not needed (ie. register time on a work item)
• May be open source or a paid product; doesn't really matter.

What would you recommend ?