Starting yesterday, Microsoft began offering a 90-day trial of Windows 7 RTM, which will be the same version that is released to the public. Though they say the offer is open to "IT Professionals", in reality it's available to anyone who wants to take it for a spin. It has a much shorter lifespan than earlier preview releases, some of which will not expire well into next year. However, for someone seriously considering an upgrade in the near future, it's a great way to get your hands on the final product.
There are other restrictions in addition to the limited lifespan, though. It requires activation within 10 days of installation, and according to Microsoft, it is only available with a limited number of licenses - in other words, it may run dry. The likelihood of that seems nil, though, as Microsoft mentioned it would be available until March 31 2010, giving people an opportunity to demo the OS even after the final product is available.
This is probably your last chance to get a free (legal) copy of Windows 7 to test with, so if you've been waiting, go for it.