TeamViewer is a free service that lets you share your desktop and connect to remote PCs. It's a lot like CrossLoop, a favorite of mine, but with a few more bells and whistles -- most notably, Mac compatibility and unattended-PC support.
Let's say Bob, your top salesperson, is in Atlanta, about to give a big presentation -- but he can't find the PowerPoint file (because, doofus that he is, he can't remember where he saved it). You, the company tech guru, head to the TeamViewer site and click the Start full version button. This downloads the "full" version of TeamViewer, which you can either install or run without installing.
Meanwhile, you instruct Bob to visit TeamViewer.com and click Start customer module -- which runs without any installation. Then you tell Bob to read you the ID and password that appear in the TeamViewer window. Enter those numbers at your end and presto: You're connected to his system. From there you can take control, chat, transfer files, and so on.
TeamViewer works around firewalls but offers very robust security. Other features include session recording, shared whiteboards, and TeamViewer Host, which you can install on PCs and/or servers for unattended access (meaning you don't need someone at the other end to read you the access codes). With Host in play, you could use TeamViewer in place of LogMeIn Free -- my longtime favorite tool for accessing my office PC when I'm on the road -- which doesn't support file transfers.
TeamViewer is free for personal and non-commercial use. It's compatible with all versions of Windows and Mac OS X 10.4 and later.