I'm constantly beset by the need to reach a computer that's not within reach: a server located 15 miles to the south, a laptop at home, or a "headless" system (no monitor attached). LogMeIn is one of the tools I rely on to complement Timbuktu Pro and Back to My Mac's screen sharing. LogMeIn's Mac OS X version, a free screen-sharing utility, lets me manipulate my computers remotely, even those inside my ostensibly locked-away home network.
The company has now announced a limited beta test of LogMeIn Ignition [1], an extension of their system, which relies on client software and centrally coordinating servers that they run, to the iPod touch and iPhone.
The App Store currently offers some remote-access packages, although these rely on VNC. VNC is built into Tiger and Leopard, but it requires directly reachable IP addresses, whether you use port mapping or have a public IP address on the computer you want to connect to. LogMeIn can traverse NAT and other obstacles to end-to-end connectivity.
[1]: https://secure.logmein.com/welcome/IgnitionPreview/