Repost:

I'm not sure if this has been pointed out in another thread or not but I thought I'd mention it since it's really helped me out.

Currently, the N900's FM Transmitter either deactivates or is reduced to an unusably low power level when and AC adapter is plugged in to the Micro USB port. This sucks since transmitting over FM burns your battery and there's usually a cigarette lighter socket nearby just begging you to charge off of while you listen & drive.

Well, I noticed that if you install fm-boost and execute it AFTER plugging in the power adapter, the N900 resumes FM transmission at the full power level of 118 even while charging!