1) Universal adapter
If you are a frequent international traveller, you understand the importance of having a universal adapter in your travel kit. While basic adapters like this one cost around Rs. 300, these days you can buy ones with multiple USB ports like the Ultra World Travel Charger with Dual USB, so you don't need to carry a charger for your phone/ tablet separately.
2) Wi-Fi Router and Ethernet cable
Many hotels across the world provide wired Internet access for free, but charge for access via Wi-Fi, often on a per-device basis. You could be a sucker and pay the extra charges, or you could go prepared by carrying a Wi-Fi router of your own, setup and ready to just plugin. The author still carries a first-generation Airport Express on his travels, and loves it, largely because it can be directly plugged into the wall. (Yes, you can do that with the current Airport Express as well, but it's not as elegant.)
You could go for something cheaper like the TP-Link TL-WR702N 150 Mbps Wireless N Nano Router, which has a small footprint and won't hurt your pocket either. Though most hotels will happily give you an Ethernet cable without charge, it doesn't hurt to carry one of your own, as they don't take too much space, especially if you can find a retractable one.
3) Extension board
If there's one thing the author has learnt on his travels, it's that the hotels never have nearly enough power points - and if they do, they are hidden under the bed, or on the other side of the room - why would I want to charge my phone next to the TV, WHY?!! Carrying an extension board fixes all this, giving you multiple power sockets (that are of the same style as your 'home' country, eliminating the need for several power adapters) wherever you want them in the room.
We recommend going for the cheap circular boards with retractable wire that you'll find at your neighbourhood shop. If you prefer having a brand attached to everything you buy, consider one of Belkin's many offerings.
4) Noise-cancelling headphones
If it seems you spend half your life in flights or at airport terminals, investing in good noise-cancelling headphones is a wise choice. Our preferred ones are the Bose in-ear noise cancelling headphones or if you are on a tighter budget the Quietcomfort 15 Acoustic Noise Cancelling Headphones are available for half as much from Amazon.
Go for headphones that come with an in-flight adapter or invest in one separately, so you don't have to use the crummy headphones the airline provides the next time you happen to find something tolerable on the in-flight entertainment system.
5) E-book reader
Throw away the dead-tree edition of your books and purchase an e-book reader to carry hundreds of books with you wherever you go with seemingly endless battery life, making it the perfect companion for those long delays at the airport.
Pick up the current-generation Amazon Kindle Paperwhite if you want the latest-greatest, or choose between various offerings by Amazon and Kobo, depending upon your budget.
6) Entertainment on the go
If reading isn't your thing and you are the kind who carries all your movies and TV shows on your mobile device, wouldn't it be great to enjoy them on the 40-inch LCD that your hotel room has? Carry an Apple TV with you and you can do just that. This experience is not limited to owners of iOS devices, as third-party apps available on the Play Store let Android devices benefit from the convenience of AirPlay.
Or you could also look at something like the Amkette Evo TV, which can connect to your phone using DLNA, if you are on a budget. Google's Chromecast is another not-too-expensive option, though it's not available in India yet.
While most hotels will have a HDMI cable (you can use the one the set-top box is connected to), carry one of your own retractable ones preferred) just to be safe.
7. Battery bank
The smartest of gadgets are nothing but dumb bricks without power. While many planes, trains, and even buses now have power sockets that can be used to charge your gizmos, there's nothing like having a portable battery bank at hand, especially if you have a long day on the road, or a week or two in the wilderness.
Choose from various capacities - a couple of thousand mAh that will charge your phone once, to 20000mAh monsters that have standby time of weeks and will charge your device several times over.
If you fly quite often, ditch your 15-inch laptop and opt for something lighter like the 11-inch MacBook Air. Not only does that mean you are lugging around less weight (those kilograms add up very quickly), laptops this size (and smaller, of course) without a hard disk-drive will save you a lot of time and effort at airport security as you don't need to take them out of the bag.
The author can personally verify having left his Air inside the cabin baggage without any incident at various airports (including the major ones in India). Of course, if you can ditch your laptop entirely in favour of a tablet (it's not as difficult as you think), your life becomes even simpler. Logitech has a great range of keyboards for tablets which can help on that front.