I need an free vpn client for mac os x leopard that supports PPTP
OS X has a built in PPTP client. http://www.bol.ucla.edu/services/vpn/pptp/docs/macosx.html#leopardpp
I use Shimo, which is really simplifies the VPN management process. Macworld did a pretty good write up on the application in Aug of '07, but much of the article is still relevent: http://www.macworld.com/article/59355/2007/08/shimo.html It does nag you asking you to license it, but the application is fully featured even if you don't license it and you can easily minimize the nag window.
Not Free, but only $9 - Viscosity VPN www.viscosityvpn.com Supports only 10.5 or later.
There is an interesting alternative to the builtin: JollyFastVNC