For this you need android phone:
1.Recharge with a Data pack  example-103
2.Open your android mobile Default Browser .
3.In the address bar type live.airtelworld.com
4.Dont close the page till 510 seconds.
5.Now your net will be disconnected and you will se a ussd massage with 0.00 mb deducted from your main account balance.
6.Now open any web browser application or any application and use net.
7. Note Before disconnecting your internet
run the steps 2,3,4,5 again.
8.Now Check Your data balance. It will not deduct.

I know this one is old but many of us don't know about this free offer.
Aircel is providing free unlimited 3g for one month. No trick its just a free offer by aircel.
But since it is free there is a limitation that is you can use unlimited 3g between 6.00 a.m to 9.00 a.m.
Check if aircel 3g is available in your area. To activate the plan you need to dial
*122*5*1# or *122*456#
After dialing the ussd code you will recieve confirmation message from aircel that your pack is activated for one month.
You don't wake up early in the morning? Don't worry just use your brain. Schedule your downloading with IDM(internet download manager) and use a software called "Reconnect" which will automatically connect your internet and you can shcedule it.
Enjoy your sleep with automatic downloading. :D

• Recharge with Rs. 303 blackberry plan
• APN : blackberry.net

Steps:

• Recharge with rs 123(1gb 3g for 30 days)
• then recharge with rs 16(unlimited whatsapp and facebook for 30 days)
• Download nmdvpn and paste this config.