APNdroid

Prevents your phone from connecting to the internet over the mobile connection - 3G/EDGE/GPRS. Saves money for the data plan and also improves the battery life. Adds suffix to APN names and types to disable them.

Buy APNdroid Pro version and to get:
* instant support directly from developer, I am reading and answering every email almost immediately (if I am not sleeping :-)
* no ads, which means clutter-free interface but also no additional internet traffic
* good feeling that you have supported open-source project which saves you some money btw. ;-)

Where to buy:
APNdroid Pro is available at Android Market at https://market.android.com/details?id=com.codecarpet.apndroid.pro

NOTE: Uninstall with enabled internet.

Quotes from APNdroid Pro users:

***** by Anthony (May 3, 2011)
Works great on Nexus S Switched from free version even though I use AdFree to block ads. Great program, use it everyday along with 3G Watchdog which automatically disables 3G when bandwidth use reaches your limit.

***** by Masood (May 2, 2011)
Used free version but switched because of him being such a nice developer.

**** by philiprking (April 30, 2011)
Works well, bought because well worth the cost

***** by iz (April 28, 2011)
Love this app, helps save tons of battery. Support you 100%. Bought the paid because the response to Airplay was ridiculous.

***** by Patrick (March 19, 2011)
Simple et indispensable pour contrôler ses coûts.

***** by Kurt (March 10, 2011)
Läuft bestens. ..SGS!

***** by Carlos (February 28, 2011)
Muito bom, acho interessante quem usa a versao free comprar a paga pra ajudar o desenvolvedor, eu fiz a minha parte. Motorola defy. 100%.