Dropbox, I was able to stop using my USB drive to carry around documents. For months, I'd even misplaced my USB drive and not felt its loss.
When I recently found it, I added a bunch of apps from PortableApps.com to it. They are mostly things I need when helping someone with their computer: file managers, anti-virus, that sort of thing.
And while I usually have my USB drive on me, I don't always. Yesterday the thought jumped into my mind that applications set to work from your USB drive are not tied to any PC. They require no entries in the computers registry, and no installation. It's like the first days of the original Mac where you could ran the application.
I copied my entire Portable Apps folder and the StartPortableApps.exe to my Dropbox, and viola, it appeared on my other PC's I have connected to the same Dropbox.
I keep files I want always available in Dropbox, now I can do it with applications too. Sweet.
And as is often the case with such hacks as this, I'm not the first one to discover this. After thinking of it, and trying it, I Googled and found others who had also thought of this.
Disclosure: The links to Dropbox here will get me 255 meg of addtional space should you signup for a Free or Paid Dropbox account via this link. I'd appreciate it if you do.