The Arduino has multiple ways to get online, from ethernet shields to ones with built-in RJ45 jacks. Then there’s all the Xbee / Bluetooth type stuff or even the USB-to-serial bridge, if you are OK involving a PC.
But “easy” wifi has been a challenge. Until now.
Someone out there built a digital camera from an Arduino and a datalogger shield. The key element is that he used an Eye-Fi SD card. Eye-Fi has built-in Wifi capability. All of a sudden, every Arduino could become a device on your wireless network! The possibilities are nearly endless, and the fact that it comes from a ubiquitous and cheap datalogging shield (these are $20) just makes it an insanely good deal.
The Eye-Fi cards are relatively cheap, too; $45 on Amazon gets you a 4GB one with 802.11n speed.
Just as a for instance, I’ve been thinking about building an ethernet-enabled IR-blaster.
Trail cam, of course. This could also fit in with any number of home automation projects — control your lights/thermostat/etc from your iPad? sure!