Here are some useful Airtel USSD codes and SMS Commands
Dial *555# : Daily and monthly sms packs
Dial *222# : Special five offers
Dial *141# : Airtel Gift services
Dial *325# : Facebook fontwish service
Dial *121# : My Airtel My offer
Dial *123*7# : To check Airtel-to-Airtel special balance
Dial *123*10#: To check Airtel 2G data balance
Dial *123*11#: To check Airtel 3G data balance
Dial *125*5# : To check daily SMS count
Dial *123# : To check Main Balance and Validity
Dial *567# : For Mobile Office Activation and De-Activation
Dial 121 : For Customer Care
Dial 198 : For Complaints
Dial 123 : To Recharge your account
Dial *566# : Musstbuy service
Dial 1909 : To Activate or De-Activate DND service
Dial *515# : Twitter services