If you're running Leopard, take a look at TimeLog 4
. It doesn't interact with OmniFocus, but it does track what application you have open and its combination of timers and nags (if you are not logging something) makes it pretty easy to use to keep track of what you do. It uses an iCal calendar (or calendars) to store its data, so you can either show your boss a calendar with your time logged on it, or export the data to a number of formats. If you're self-employed or you have different categories of billable hours at work, you can enter your base rate for different kinds of tasks, and you can add other costs as appropriate, and TimeLog will calculate the value of the work you've done and help you generate invoices.
TimeLog 3 (for Panther) will do a lot of that but it has some annoying bugs, such as quitting and relaunching iCal every time you record a new entry.