Which technology allows a user to securely access a company lan across a public network?

Vpn

Agree with Arun.. VPN is your best solution. And since you have mentioned a Company Lan, then one of your best options are WatchGuard if you heard of them, its multi-layer business router/firewall that supports different VPN options, depends on your router's/firewall model, you can get different subscriptions for3 years that can beat any other company in the market such as cisco :)

Regards

VPN  allows users to  have secure access with comapny lan using public  network

VPN هي تقنية تسمح لك بالاتصال بين شبكتين عبر شبكه عامه 

استخدامته 

1- remote access VPN 

2- site-to-site VPNs 

تقنية VPN توفر 

1- autentication تحقق ( passwords - digital certificates ) 

2 - DATA confidentiality سرية المعلومات ( diffie-hellman key Exchange باستخدام خوارزميه DES ,3DES , AES بالاتفاق بين الموقعين )

3 - DATA integrity صحة المعلومات باستخدام خوارزميه hash md5 -SHA-1

VPN

Use a VPN (virtual private network).

VPN

VPN

IP tunnels are the most secure one used l2p, gre(generic routing encapsulation), IPsec(private-to-private network)

these are secure VPN's

ISP Admin as long as you know the know the user and passworld you may access it.

VPN...

You can try OpenVPN Server and client here

http://openvpn.net/howto.html