I first picked up MasterLock's carabiner-style Backpack Locks, above, years ago in preparation for a backpacking trip across Europe. Lightweight, slim and unobtrusive, I had a couple of them hanging off the loop of my backpack, and I could quickly use them to shackle one thing to another; if I wanted to keep my camera out on a picturesque train trip but was dozing off and worried about lightfingers, I could affix my camera to a belt loop; at sidewalk cafes I could attach my day bag to a chair arm and not have to worry about snatch-'n-grabs. And the design makes it easy to hook and unhook them, making my little security precautions quick afterthoughts rather than paranoid productions.

In the years since MasterLock has been steadily releasing locks in creative form factors, like this sleeker, narrower version of the Backpack Lock:

They've also got an ADA Lock, this one made to be easily grippable for those with manual difficulties. It also auto-scrambles upon locking, so the arthritic user doesn't have to bother with grabbing and turning the thing any more than necessary:

Their shrouded series of locks, designed to keep bolt-cutters away, provide an interesting variety of industrial designs:

MasterLock is confident enough in their bad-ass Street Cuffs, designed to securely shackle bicycles, that these come with a $3,500 anti-theft guarantee:

The Retractable Cable Lock is a handy way to fasten a bunch of things together (a group of duffle bags, for instance) without having to carry a separate cable:

The most amusing lock of theirs I found is the Speed Dial series; rather than numbers, your combinations are made of "moves," i.e. Up-Up-Down-Left-Up-Right-Left, at a length of your choosing. Seems we finally have a lock designed for the PlayStation generation.