My EDC Palm PDA, which is (with some accessories) for:
(not in any particular order)

• eBooks reading. Helps to spend time productively when there is nothing else interesting to do (like taking a bus to work).
• Day planning assistance. Not just following the check lists but recording voice, images, video, handwriting notes/diagrams for future processing.
• Copies of all of my documents ready to view and print. MS word, Pdf, Excel, scanned images (personal, financial, technical, work related, etc...).
• Information: logins/pins/passwords/codes/keys data bank, address book (personal and white pages), calculators, units converter, vocabularies, encyclopedias on many subjects, etc...)
• Navigation: GPS, buildings plans, transportation maps (and schedules), travel guides, etc...
• Communication: email, IM, social networks (phone is separate device).
• Entertainment for me and my family: music, video, solo and multiplayer games.
• Information exchange (use it as a big flash memory stick).
• Programming in C++ (yes, my work is one of my hobbies too).
• Flash light. It's not very bright tactical light, however I have a custom application which makes it much more versatile than any advanced multimode flash light (for example I can type a text and it will flash it in Morse code by red colored light).
• Universal IR remote control (however the readily available device database I have is only ~500 devices so far).
• Send and receive faxes (yes, some people are still insist on using fax to communicate )
• Serial protocol device controller (I can rig it on top of Roomba vacuum cleaner and force it to go and trigger an explosive for example ). Seriously it helps me very much at work to simulate various devices which I don't have readily available at hands due to the budget constraints.
• Wireless spy camera (recording or broadcasting in real time).
• Internet, where available (WiFi, 3G over BT). There are a lot of fascinating things going on in the universe (just skip the politics, economy, and celebrities).

All in the palm of my hand for 10 hours without recharging - enough to help me through the day and beyond.