Android phones, apps and tablets

is there anyway of finding out data usage?
Im on tmob and have a Desire (mk1)
looking at new contracts and 500mb seems to be the norm for data, when i took my contract out it was 3gb limit.
i havent got a clue how much im using, so ma worried that if i get a 500mb limit i might reach/ breach it regularly.

I've been using Onavo and have found it excellent in tracking data usage, can also assign limits and do things like make certain apps wifi only