TeamViewer is a popular GoToMyPC-like app that was Windows-only until May 28th, when the Mac client was announced. The application is free for personal use, which is nice for those of us who want to get "Back to our Macs" but can't get BTMM to work or don't have .Mac accounts.
I gave the freebie service a test last night, accessing my home iMac from my MacBook Air over a Sprint Mobile Broadband connection. Setup was simple: I installed the application on both Macs, then wrote down the ID and password generated by TeamViewer on the iMac. I left TeamViewer running on the iMac, then went out to dinner.
While enjoying a few beers, I popped open the MBA and fired up TeamViewer. I told it to connect to my iMac, gave it the ID and password, and was rewarded with complete control of my machine at home. TeamViewer works well for remote control, file transfers, and blasting presentations out to co-workers. For commercial use, TeamViewer sells licenses varying from $249 for six months, or an unlimited license for $1399. If you're a system admin who needs to control both Macs and PCs remotely, TeamViewer may be the app for you. Download TeamViewer here and see if it works for you, too!
Note: Quite a few commenters (and our own Mike Rose) also recommend LogMeIn as a free/paid option for remote control of both Macs and PCs.