Hi,
One of the simplest and least likely to cause problems, is a [custom stylesheet|http://www.floppymoose.com> that attempts to not display typical ad content.
There is also a 3rd party product called [SafariBlock|http://www.macupdate.com/info.php/id/19202] that may or may not work with Safari 3 on Leopard. Have a search on macupdate/versiontracker.com also, there may be more alternatives.
Bear in mind products that use the Input Managers (as SafariBlock does) sometimes cause problems for subsequent Safari updates. So, if you do use it and Safari starts to crash down the line, uninstall it to see if it is causing the problem.