When there’s no space left in the memory card of your digital camera, you normally have to hunt for a USB cable to transfer all your pictures and videos from the camera on to your computer’s hard drive.

With Eye-Fi, the process gets a little easier as this affordable cards can turn your digital camera into a wireless device.

Eye-Fi is just like a regular memory card but with a built-in Wi-Fi chip so you can wirelessly transfer photographs and videos from a digital camera to your computer over an existing wireless home network* without requiring USB cables. Here’s a video of an Eye-Fi memory card in action.

Other than transferring media from the camera to the computer, you may also use an Eye-Fi card to post pictures /videos on Flickr, YouTube, Photobucket, Facebook and host of other social sites even when your computer is off.

Now if you think it makes sense to buy an Eye-Fi card this holiday season as a gift or for your own self, here’s a sweet deal for people in US and Canada. If you buy a 200 GB account for your Gmail and Picasa Web Albums, Google will send you a 4 GB Eye-Fi Home video card for free (update: offer has expired).

The regular online price of the Eye-Fi 4GB card is around $70 but if you buy the same 4GB SD card though Google, you need to pay only $50 - that’s a saving of $20 and you also get 200 GB** of storage space for your photos and videos on Picasa Web Albums.

PS:If you don’t have a wireless router at home, you can still use Eye-Fi to transfer photos wirelessly by setting up an ad-hoc wireless network on your Windows or Mac OS X machines but the 4GB card that is part of the Google offer doesn’t support ad-hoc networking.

[**] Google will charge you an annual fee of $50 for maintaining your 200GB Picasa account but you can always downgrade or even cancel your account later and still keep the Eye-Fi wireless card forever.

Related: Online Backup for your Photos [Comparison of services]