Something I always keep in my laptop bag is a Liberator power splitter.  All this does is split one power outlet into two, with a little bit of extension cord thrown in for added convenience.  You can find a wide variety of such things here.

Always Enough Power For You

So, what's the big deal about having one of these?  Well, you could throw a whole power strip in your bag, but that's more bulk and weight.  If you don't mind that, then go for it.  Assuming you don't want to carry that, though, the benefit of a Y-splitter is that if you walk into a room (or conference session, where power outlets are always in short supply at geek conferences) you can simply unplug somebody's laptop (ask first), and then plug in the Y-splitter and plug them back in.  Similarly, at the airport, if you're lucky enough to find a power outlet but it's in use by some folks charging their cell phones and iPods or using their laptops, you can unplug them (ask first...) and then plug yourself in along with them to your Y-splitter.

This has allowed me to get access to power outlets on many occasions, and is well worth the < $5 cost of the Y-splitter.