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.
I've been a Logmein user for several years now. What initially got me into it was the FREE remote access that was easy to install, setup and use. It was first used as a backup to the standard RDP remote access, but now its the first place I go. I put on the hundreds of computers I support on it over the years and it has not let me down.
They made a great product even better by introducing the Logmein iPhone app , which gave me even more flexibility. Now its not even essential for me to walk around with a small notebook to handle support requests.
I use logmein every day.
Anyway, check out the PCMag review here.