While Microsoft has its own solutions for running IE6 apps on Windows 7, see their whitepaper Solutions for Virtualizing Internet Explorer, they can be somewhat overkill and expensive to manage.
Dependence on IE6 for legacy web applications is often cited as a reason preventing an upgrade from XP to Windows 7, which provides improved security and easier implementation of least privilege.
Unibrows, currently in beta, is a product that runs as an add-in for IE8 and is triggered automatically according to rules configured by a system administrator in Group Policy to use IE6 code and display the page in the current tab.
Unibrows will cost $5 per user a year and is due for release at the end of November. Definitely a solution worth looking at if you want to upgrade to Windows 7 but can’t relinquish support for IE6.