With one of the 10.5.x updates, Apple removed the hidden screen sharing preferences that gave you a toolbar item to enable and disable keyboard and mouse control when viewing a client computer with Screen Sharing. This feature is still missing in the Snow Leopard version of Snow Leopard.
But, the method call to turn that feature on and off still exists in the application itself. I have made a quick Screen Sharing plug-in that adds a menu item you can use to toggle this feature. Note: This is a SIMBL plug-in, as I don't currently know of a better way to inject this menu item/method call into Screen Sharing. If you don't like the idea of loading third party code into an application at runtime, then don't use this.
The code is completely open-source, so you can inspect it if you wish. I also have a pre-compiled bundle ready to drop in. All information on download/installation is available on the above-linked GitHub page. Any issues you may find can be reported on the issue tracker there as well. Hopefully this will be helpful to those, like me, who aren't looking to spend big money on ARD for just viewing a couple home computers or a co-worker's screen.
[robg adds: I tested this and it worked as described. More personally surprising was the existence of a version of SIMBL that works in Snow Leopard; I didn't think this was going to be possible, given everything I'd read.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20090923092314664