Imagine a gadget that fits in your back pocket and lets you surf the Web anywhere, write documents, make VoIP calls, watch movies, and listen to your entire music library. That’s not exactly what Nokia had in mind when it released the 770 ($360; nokia.com), a PDA-size Internet tablet with Wi-Fi and Bluetooth. But because the device has an open-source operating system, anyone can build new programs for it, endowing it with nearly endless functions (we’ve nicknamed it the HackBerry).