Cisco VPN Client 4.9.01.0100 works for 10.4 and later, including 10.5.
It can be hard to find Cisco VPN client downloads, Andrew got this one for me.
Update: I couldn't get the image to download fully, and now it's offline. Comments tell us Cisco's distribution policy has not change, this was an unauthorized image and it's probably been removed.
I was able to download an image through my university account. It took 3 tries, I kept finding older versions on various U servers. The official site (UMN authentication required) had the version I needed: Cisco VPN Client 4.9.01.0100
I think for most people that's the best way to get an updated VPN Client -- find someone with access to an university account and ask them to get the client. Just be sure they know what the latest version is! I suspect many universities have multiple distribution sites.