Mallet
Something like a hammer but with a rubber or rawhide head. Used for whacking things. I have found the ones with a rubber head in dollar stores. For most applications, you can use a hammer instead, but remember that hitting metal with metal will wear out your tools faster.

Leather Punch
These come in two varieties, a 'rotary punch' which is used by squeezing it (much like a paper hole punch), and a 'standard punch' or 'punch set', which you use by whacking it with a mallet. Both come with several bits to make various sizes of holes. The rotary punch has them all on a wheel, and you rotate the correct bit into place, while the punch set comes with a set of loose bits which you can screw into the handle. Rotary punches are often cheaper, but can be hard on your hands if you have to make a lot of holes, or are punching through thick material, and it can be hard or immpossible to use it to make holes in the centre of your material. The standard punch needs to be used in conjunction with some material under your project which will not dull the bit when it impacts it. Polystyrene mats for that purpose are available at leather stores, or you can just use a piece of scrap softwood or a -very- thick piece of scrap veg-tan leather - not as good, but better than driving the punch directly onto stone, metal, or your dining room table.

Tin Snips
Essentially sisscors designed to cut through thin sheets of metal, they are also effective at cutting plastic, heavy wire, and leather (though you may get a cleaner cut in leather by using a knife).

Box Cutters
Get a good, solid knife at a hardware store (sometimes you can find a decent one at the dollar store), not the cheap keychain sized ones. Very useful for cutting through thick leather, cardboard, foam, etc. Get lots of spare blades and replace the blade when it starts getting dull. Dull blades lead to ragged cuts, and are more likely to slip, leading to injury, because you need to use more pressure.

Scissors
If you can, have several pairs. You will want a small pair for cutting thread and ribbon, a cheap pair for cutting paper, anything with metallic thread in it, and miscillaneous materials, and a VERY SHARP pair for cutting fabric and thin leather. Never ever use your fabric scissors to cut paper, you will dull them instantly. Your fabric scissors should only be used to cut fabric, thin, soft leather, and hair. Michaels occassionally offers free scissors sharpening.

Needles
There are several different kinds of needles available, for both hand- and machine sewing, and you should use the appropriate one for the material you are sewing. Multi-purpose needles are good for most fabrics, but not all. Delicates needles are thinner and are good for thinner or dellicate fabrics. Needles for knit or loosely woven fabric, especially wool, have a rounder point rather than a sharp point. Leather needles have a different cross-section, and actually have a sharp edge near the point, to cut through leather rather that just punching through. Leather needles also work very well on thick denim. There are also hand-sewing needles meant to be used with pre-punched holes in leather, which are thicker and don't have a sharp point.

Seam Ripper
This is tool is designed specifically to cut the threads in a seam that has already been sewn. You can almost always get away with a small pair of scissors if you are careful.

Setters for rivets, gromets, snaps, etc.
You will need a specific setter for each size and type of hardware you use. Most setters require you to also own a mallet. Often you can get a kit that comes with a setter, instructions, and some of the hardware, then later when you need more grommets, or snaps, just make sure you get the same size. Setters for basic rivets are designed to maintain the slight curve of the top cap; if you don't mind them ending up flat you can forego the setter and just whack it directly with a mallet. For fancy rivets you will probably need to use a setter. You will always need a setter for snaps and grommets.

Awl
This is a tool designed to punch tiny holes in thin leather, or mark where a hole should go in thicker leather. You can usually use a thick needle or small nail instead, you just don't get the nice comfortable handle.

Sewing Awl
This is basically a needle and awl combined, used to punch holes as you sew, with thick thread that would otherwise need pre-punched holes. It's a bit tricky to get the hang of at first, but they generally come with instructions, and it is much faster than punching holes and then sewing. One word of advice - don't use waxed thread with this tool, it tends to gum everything up.

Tailor's Chalk
This is usually blue (as that is visible on the most colors of fabric) and comes either as a rectangular piece, or something like a pencil. It is used to mark fabric temporarily, and the marks can be removed with an eraser, stiff brush, or washing. On some fabrics you can use a pencil instead, and a pencil actually works better on veg-tan leather. If you are making marks on the back side of your fabric or in the seam allowance where they will never be seen, you can get away with using a pen, but test it on a scrap piece to make sure that the marks are not visible from the other side of your fabric.