See, Google's Android OS is open-source, and developers have wasted no time porting it to work on various WinMo hardware. All you need is a compatible phone, a microSD memory card, and good instruction-following skills.
What phones are compatible? Mostly models made by HTC, though a handful of Samsung phones qualify as well. (Click the above link and scroll down about midway to find a complete list.) The best candidate is the HTC Touch (also known as the HTC Touch and Verizon xv6900), which has "100% of the hardware features working."
Are there risks involved? Of course. Once you install Android, you probably won't be able to restore Windows Mobile. And at least one commenter reported a "bricked" phone -- not a good outcome.
Even so, I desperately wish I had an old WinMo phone lying around, because I'd love to give this a try. The Android OS is light years ahead of Windows Mobile, and it could breathe new life into an otherwise crummy handset.
If you decide to give it a whirl, please, for the love of all that's holy, hit the comments and let us know the outcome. Photo by The How-To Geek.