Well, I have a large piece of property (21 acres), & so usually drag my trees into my woodlot & let the wildlife use them for shelter. I suppose eventually once all the needles drop off of them I'll either burn them or chop them up to use as fire starters/kindling for my wood stove.

I cut off the branches and throw them in my front flower garden. As the dahlias and other tall plants come up, I use these branches as natural looking plant stakes.

I leave mine standing inside the house until it is dry (started doing this because I rarely put it up until a couple of days before Christmas and I'm not ready to take it down very soon, want to enjoy seeing it for awhile.)

While the weather is still cold outside, and there being little or no yardwork for me to do, I use the Christmas tree as a pruning project. I start with hand clippers and trim off small pieces, about 1 or 2 inches long, putting them all into a large paper yard collection bag. Once I've got all the small stuff removed, I use loppers to cut off small chunks of the main branches, again putting them in the bag. I don't do this all at once, but at intervals over a few weeks' or even months' time, maybe while listening to music or a boring television show. At some points the tree looks pretty hilarious, so I sometimes invite people over to join in the fun. Depending on the size of the tree, I end up with a bag or two of finely chopped pine needles and pine branch mulch. All the mess stays in one place, no needles where the tree has been dragged across the carpet to the front door. When the tree is nothing but a naked trunk, I sweep up all the loose needles from the floor under it. And when spring comes, I use the mulch in one of my flower beds where I'm trying to build the soil's tilth. It makes very effective mulch, with a wonderful smell of sweet rotting pine. Bees are very much attracted to the smell, as I discovered one year when I put the bags out on the front porch. Now I leave them indoors, and they provide a room-freshening scent - there's no real rotting going on, it's a dry process.

The stripped trunk can be used to build a trellis for vines by tying strings to the top and pinning them down at the ground, May-pole style. Or, depending on its size, for any number of other outdoor projects.

I've also heard of people "planting" their used Christmas trees in the ground outside, and hanging suet balls or other wild bird treats from all the branches.

Some local governments collect spent Christmas trees to use for fish habitat. They simply drop them into park ponds, where they sink to the bottom and provide hiding places for the fish.

Last year's tree was taken apart and used for mulch, but I built a cat tree out of the one I had two years ago.
I stripped off the limbs and bark and put them outback under the deck so I could water the flowerbeds and not get muddy. Then I took apart some old shelves that were sitting in the garage, and attatched a 3' base with alot of wood glue and a 8" bolt right up the middle of it. I took the shelves and attatched them with 1" metal braces (2/shelf), and 2" metal brackets (2/shelf). I stained the whole thing including the hardware and carpeted it. Home Depot in my city was getting new carpet displays and were selling their old ones (about 2' by 1 1/2') for a dollar a piece. It was a four dollar cat tree! Cats like it anyways.

While looking for suet feeders online I got a new idea for using the trunk of last year's Christmas tree (still standing naked in my living room, I should be ashamed to say). I can hardly wait for the weekend to get started on this project! My tree trunk should yield half a dozen or more suitable pieces for suet feeders like this, and I'll take the extras to our next plant trade.

Here is a link that might be useful: log suet feeder

Friend who sells Christmas trees passed this along four-step approach:
1. Christmas decoration
2. Move outside - can use for bird protection in wintry climes.
3. In Spring, plant morning glories to clamber over it. (Did this and it looks great.)
4. Next winter, spray paint it white, put on lights and it makes a nice yard decoration again.