For me, its gotta be Logmein Ignition. I have remote access to hundreds of PCs anywhere with WiFi or ATT 3G coverage both on my iPhone and in the future, a 3G iPad.

$29.99

I imagine in the near future medical health records are going to be huge! Artists may actually do some serious work, and photographers like myself might just do some photo editing and portfolio / shoot presentation and sales directly on the iPad.