OS X 10.5 includes a very handy Screen Sharing application for controlling remote Macs. It is invoked via iChat's Share Screen feature, as well as a Share Screen' button on locally-discovered Macs in the Finder. But, if you're like me, 90% of the time I just want to enter the IP address of the machine to control and be on my way. Luckily, the Screen Sharing program is able to run independently, and will ask for the IP to control upon launch.
The application is fairly well hidden (which is what makes this a hint!), but you can find it here: /System » Library » CoreServices » Screen Sharing.app. I suggest making a dock shortcut to it.
[robg adds: An anonymous contributor also pointed out that you can access the program via the Finder's Go » Go to Server menu. Choose that, and type vnc://220.127.116.11 in the input box, where 18.104.22.168 is the computer's IP address or DNS name. Screen Sharing will launch and prompt you for a password.]
Mac OS X Hints