I’ve recently discovered DropCopy, from 10base-t Interactive. This little utility’s "window" is a small translucent dark spot, rather like a hole, that sits behind all other applications (and behind your desktop Finder icons). Drag a file or folder onto this hole, and a menu appears next to it, listing the names of any other computers on your local network that are also running DropCopy. Continue dragging onto an item of that menu, and the file or folder is copied to that computer.
You could use Personal File Sharing to accomplish the same thing, of course, but DropCopy feels far more lightweight: you don’t need to turn File Sharing on, you don’t need a username and password, you don’t need to log on or open any remote Finder windows, you don’t need to worry about permissions. Instead, DropCopy uses Bonjour (formerly known as Rendezvous) for auto-discovery and data transfer. You could use iChat, but you’d need to arrange multiple screen names to avoid the "multiple logins" problem, and you’d have to be at both computers at once (one to send the file and the other to accept it). With DropCopy, you just send a file into the hole and it’s on its way. For just popping an occasional Finder item over to another computer, DropCopy is simply perfect.
DropCopy also lets you post a text message dialog to another computer, and you can even fetch the contents of another computer’s clipboard. (Back in the old Mac OS 8.6 days, I wrote some gruesome AppleScript tools to accomplish that.) If you’ve got more than one computer on your local network, even if there’s just one human user, you’ll probably find DropCopy a huge time-saver. The interface is delightful (I love the animation as the hole darkens and lightens while a file passes through it), and the price (free!) is right.