I bought an acorn squash on a whim, and now I have no idea what to do with it. Help?

We do ours 100% in the microwave. Stab it with a knife a few times, microwave it for about 5 minutes, then cut in half and scoop out the seeds (careful, it will be hot). Put cut side down on a plate and cover with plastic wrap (make sure it is microwave safe) and put it back in the microwave for another 5-8 minutes (depending on how big of a squash it is). Serve with brown sugar and butter. Easy and quick.

Quick tip: If you're having a hard time cutting into it, poke some holes in it with a knife and nuke it in the microwave for a minute or two; just long enough to soften it so it's easier to work with.

And another classic idea that never get's old; roast it with olive oil, S&P. Brown some butter in a pan with some whole (or chopped) sage leaves (or rosemary) and just pour it over the roasted squash. Or, if you eat pork, replace the olive oil with rendered bacon fat instead (my preferred method), then drizzle the brown butter over it. ...Maybe a drizzle of aged (or reduced) balsamic to finish...

I love stuffing winter squash halves with a wild rice-sausage stuffing. Wild rice, leeks, mushrooms, sausage, etc.

I proceed as detailed above (carefully cut across the equator, scoop, slice bottom so it will sit without wobbling. Then I roast them face down at 400 degrees till they are fork tender. I flip them onto a plate, season with salt and pepper. I fill them with scrambled sweet sausage (scrambled with onion, mushrooms, and garlic, till beautifully browned). Then a little drizzle of maple syrup. My daughter named this dish "Squasage." It is a seasonal favorite.

All good advice so far per above. I hope you have an axe or a chain saw handy because those little bast...s can be difficult to open up.

Slice into wedges (removing seeds), toss with salt, pepper, fresh thyme, and oil. Roast until tender.

This squash with chili-lime vinaigrette recipe is fantastic:
http://smittenkitchen.com/blog/2006/10/unflinchingly-good-things/

Cut it in half (carefully, knives can slip easily on winter squash skins), scrape out the seeds with a spoon, fill the cavity with a little butter and maple syrup or brown sugar, place in a baking pan and roast at 350 or 375 until tender. Test with a fork or the tip end of a knife.