I was recently asked this very question and I responded with "Windows Server 2008/2003/2000 and Windows Vista/XP" provide excellent built-in Virtual Private Networking features that are standards compliant and assuming they are correctly configured can be highly secure too. There's no additional license cost required to use Microsoft Windows VPN. In addition Microsoft offer a product named Intelligent Application Gateway which provides a high-end alternative to traditional VPN - it's part of the Forefront suite of products. IAG provides the means to publish applications from your internal network to authenticated users outside while adhering to a series of customisable rules specifying what types of authentication, inspection and access is allowed.
The next question of course is "how should I set up the VPN Server and Client/ are there any good guides?" - there are some excellent resources including step-by-step configuration guides on TechNet - just click here to access them.
Note: the URL is really easy to remember as it's just microsoft.com/vpn !