You don't say what phone you have, but on standard Android if you go to Home -> Menu -> Settings -> Applications -> Manage applications (wait while the list and icons load). From that screen you can see how much memory each app is taking up.
Find some big ones (Browser and Market would be a good examples), press on it, and you will see the amount of memory that is being used by the cache on that app (assuming it has one). Press on the "Clear cache" button to empty that. DON'T press the "Clear data" button unless you want to lose all the settings and stored data for that app.
