Thinking of an Airport Express...

Yep. About the same size as the MagSafe adapter. It doesn't just boost WiFi, it can be your only wireless router.

http://www.apple.com/airportexpress/

 

Plug in any USB printer and you're set to go. It can extend an existing wireless network, or plug an ethernet cable in from any router and it can create a wireless network. It's my only wireless router, creating a wireless network from my cable modem. Any Mac or PC you choose can join the network.

 

Your left back pocket.

Save a few dollars: http://store.apple.com/us/product/FB321LL/A?mco=MTY3ODQ5OTY

 

Well hold on there, the APE is designed to extend WiFi networks from Apple products. There are several "standards" for doing stuff like that and they aren't all compatible. The APE may not be able to extend your network if you don't have an Apple router. And even when it is compatible, keep in mind there are tradeoffs like slowing down the network speed. For me, I use my APE purely for the AirTunes functionality. That's worth it alone.

 

Using the AirPort Admin Utility to create a WDS network with multiple base stations
AirPort Express: What is client mode?
AirPort Express: How to join an existing wireless network in client mode

 

Yes, this is true. I bought my Express thinking that it would extend my home network (I use a linksis n-router) but it simply won't work. It'll only extend if you have another Apple router. Still, I'm gonna keep mind for whenever I decide to travel.