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You can do that too, but such an app can be very helpful to see the situation in one overview, check what is on SD card, what can be moved and clear all cache. I find it useful.Why do you want an app when you can go into settings, manage apps and move what you want to sd card
What phone are you using? What OS version?
If you have Froyo, then I think App2SD Free is the best application for that.
Link: https://market.android.com/details?id=com.a0soft.gphone.app2sd&feature=search_result