If you want to make commercial use of Google Calendar, including but not limited to selling or distributing Google Calendar for payment, you must enter into an agreement with Google or obtain Google’s written permission in advance.
I spent some time looking at the API, and I’m confident that it won’t be too difficult to re-use a lot of SyncBridge to develop this functionality.
I’ve also spent a bit of time looking at the 30 Boxes API. From what I can see, there is no way to request a list of calendar events based on their last modified date, which means that providing two-way synchronization would be a very costly exercise (you’d have to download all your calendars every time you synced).
Unfortunately, Yahoo! don’t seem to have an API for their calendar :-/