Windows only: Joining the ranks of Crossloop, iRemotePC and LogMeIn, TeamViewer lets you connect to other PCs for remote access, tech support, file sharing and more.
The tiny setup program gives you the option of configuring your PC in host or client mode, or skipping installation altogether (in which case TeamViewer runs without installing anything). I chose the latter option on two PCs: one running XP, the other, Vista. From there I simply entered a numeric ID and password generated by the software, and presto: Instant remote access. In addition to controlling the other PC, I could transfer files and chat with the other user (myself, in this case). By enabling server mode, I was able to switch directions and share my desktop with the other PC—neat for showing presentations, demonstrations, etc.
TeamViewer works around firewalls and promises 1024-bit RSA private/public key exchange and 128-bit RC4 session encoding. In other words, it's pretty secure. It also has at least one advantage over each of the three aforementioned remote-access programs, so if you're interested in connecting to another computer from afar, give TeamViewer a try. The program is free for personal, non-commercial use, and it requires Windows 98 or later.