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Donate it. There are dozens to hundreds of organizations to which you can donate your phone. Particularly those that help victims of domestic abuse...

While this seems like a kind gesture, I advise everyone I know against this because what happens is these phones are turned off and can only dial 911. They often fall into the wrong hands and are used to make fake 911 calls. I work in Law Enforcement and I've seen all too many times that a fake call of an officer down or shots fired comes back to a disconnected 911 cell phone. I'm not saying everyone does this, but it does happen. My vote is for a paper weight cause that's probably all it was ever good for anyway.
 
Use it as a wait service tray for serving food at a restaurant for little people.
 
Does it have WiFi ?

I used to use WinMo phones with WiFi around the house as portable home Slingplayers. Porch, sofa, bathroom. Great for the kids, too.

Can also put Skype on them and use them as home WiFi Skype handsets.

Also the ones with handwriting recognition are useful with kids. My daughter learned a lot about writing that way when she was three or four. (She had to write well enough with the stylus on the screen for it to recognize the letters.)

Even games are good on them; keep it around the car if you have kids.
 
Unless you can get a worthwhile price then I would keep it, either as a secondary phone for someone else or as a backup for when the new iPhone comes out. You could pre-order the iPhone 5, sell the iPhone 4 after taking your SIM out and put that SIM into your WiMo phone until you get the new iPhone.

I am pretty sure an iPhone has a better resale value.

EDIT: I guess that won't work with the SIMs being different. Oh well.
 
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