This is a short list of props that I especially like, and other favorites will be added soon. Browse through the various ideas and see what catches your eye!

Indoor Decorating Ideas

Cut out different sizes of bats (and other characters if you wish) out of black construction paper, felt, or other materials and lightly tape them to the inside of a lampshade that will allow them to show through when the light is on.

Cut out sinister silhouettes from dark paper to place in windows. At night they show well with the lights on and create a great effect.

Take plastic "candle" lights, the kind found in stores around Christmas that have three or four candles attached, though an individual would still work well. Paint the base of it black, silver, or whatever color you choose. Paint the molded plastic "drips" red to look like blood, then screw in flickering lights.

If you have a deer head (or anything else along those lines) hanging on a wall, affix a small black witches hat to it's head. Tie a small loop at the end of a piece of black thread, tie a plastic spider (a spider ring or the kind that comes with the fake webs) to the other end, then fit the loop over the point of the witches hat. This looks best if the hat is sticking out rather than straight up and down so the spider hangs nicely. I have found a couple paper clips and a plastic thumbtack work great. Attach one paperclip to the back of the hat rim, bend the other into a hook shape (use more paper clips if necessary) and hook it to a tack pressed into the wall just above/behind the animal head. *A long-horned steer skull creates a wonderful effect!

Basic Outdoor Suggestions

A fogger
Nice small fog machines, perfect for home use, are easy to find around Halloween and are normally reasonably priced. Two bottles of fog "juice" should get you a long way on the average Halloween night. Though expect any Halloween on a Friday or Saturday to use more. If you can't get a fogger, opt for dry ice. Just keep it in a cooler prior to using and never pick it up with your bare hands. Place some in a bucket of water and add to it now and then. Note, it does not work as well as a fogger, especially if it's windy at all.

Black trash bags and mulch or chips
If you have any creatures popping up from the ground or crawling out of graves, it works well if you surround them with a bit of mulch or chips. Cut open a black trash bag to lay down under your prop and spread the much around it, covering all of the plastic bag. This creates a more effective look and makes it easier to remove a pile of mulch from your yard.

Large insulation boards
If you have a way to prop them up and secure them, these make excellent walls to help conceal a few surprises. If your yard is large enough to make a path through it, add a few walls to create a darker area. Place a black or red light in this area to light up whatever horrors you have placed there.

A shredded curtain
In addition to the walls, you can add a curtain made from a large piece of black material. Cut it up in strips (be reckless in the cutting), varying thickness' between 2-3" is usually good. Leave at least 6" at the top to attach it to a dowel. Fold over the end, stitch it and slip the dowel into it. (Or use a staple gun or other faster way if you want to.) If you have put up walls that create an entrance to your path, you can affix this curtain across the entrance to make your darkened area spookier. (Be sure the curtain does not touch the ground. You do not want anyone stepping on it and accidentally tearing down some of your hard work.)

Small lanterns and things
Hung them on tall hooked hangers, the kind that often hold plants in hanging pots. All sorts of shapes/characters with tea/votive candles add a great effect.

Spooky Music
Tuck a CD or cassette player in a hidden spot that is still easy to get to. Be sure all electric cords are away from wet areas, and out of the way or secured so that no one will trip over them.

Life Size Monsters
Stuff old clothing with sheets, old news paper, other cloths, whatever suites you. Prop up some place, sit in a chair, or tie to a tree. Stuff a monster mask and pit it on. If a mask only covers the face, use a shirt or jacket with a hood if possible that you can stuff as well.

Props to Build

Bottomless Pit

One of my personal favorites and is always a hit attraction. It is done with a simple wood box a light and tow mirrors, one regular under the box, and a two-way that lays on top of the box. (Mirrors are put in thick wood frames.) When the light is off, the two-way does not permit any light inside so it looks pitch black. It is very impressive when suddenly turned on. It is recommended that you use foam board to create rocks or bricks to glue to the inside. Spanish moss bought at a craft shop is also a great touch to the inside. I recommend the box being 2'x2', though some people have made it bigger. Keep in mind the weight and storage space when deciding on the size. I do NOT recommend actually attaching the two framed mirrors to the box as some people do, for the safety sake of storage for the rest of the year. Make the frames wide enough and they should stay put just fine. Do use caution that young children don't hang on it. These two links have the best instructions I've seen:
Bottomless Pit
Another Pit example

Tombstones

A must for Halloween. What spooky setting doesn't include a grave yard? Tombstones can be made using thick foam board. It is easy to cut and carve. Inexpensive tombstones can be made out of large pieces of thick cardboard cut into the appropriate shapes and painted, including a darker edge on one half to simulate depth. A little bit darker and lighter paint added later with a sponge (use it very lightly, do not press!) is recommended to create a textured look. Carefully nail wood stakes to the backs. This can also be done with plywood. Because of the weight keep the plywood thin, but still thick enough so it will not easily crack. This link has a great example for carving images into the foam:
Foam Tombstones

Cemetery Fence

No cemetery is complete without a fence, and occasionally keeps people out of areas you don't want them in. Though many people have lifted their young children over it to take their picture standing in my cemetery. Small pieces of real wood fencing that doesn't stand very tall can be used if you fix pieces to other supports to keep it standing. This is only recommended if you actually have a place to store it. A lighter and more sane alternative is to use narrow pieces of PVC pipe. This site has great instructions on how to do this:
Cemetery Fence
Columns
Another Columns example

Guillotine

13 Hour Clock

Other Prop Links

HuntersHangOut.com

Paul's Halloween Props