What are the 10 best exercises to get a six-pack? (firm stomach muscles)
And briefly describe the exercises.
Thanks

1. Stop buying and "using" six-packs.  

(Sorry, maybe you don't anyway, but I just couldn't restrain myself. ;)

2. Situps, and more situps; straight forward, diagonally  - left elbow
to right knee and vice versa -  and more situps.

Someone told be that it can be bad for your back to do them with your
knees flat down on the floor, so move your bottom closer to whatever
is holding your feet down so your knees are bent.

3.  For relief, do leg lifts.

Repeat steps 2 and 3 four times to get the ten best exercises.

If this gets boring, you can also do pushups.  A sixback stomach looks
better with good pectorals.  And after doing a dozen or so, you can
jump your legs up to your chest a few times, and then do some more
pushups  - before you collapse and roll over and start with leg lifts
again.

No, this isn't terribly serious, and I am sure someone can provide
better information and suggest expensive machines to train on, but it
is not incorrect.

Do I?  Well, sort of.  I am about to do a very mild situp from my
laid-back swivel chair and go see what a sixpack looks like.  ;-)

I can't come up with 10 different exercises, but here are a few that i
know are quite effective:

1. sit-ups, but not the ones you usually see people do. Here's the trick:
   Lay down on the floor on your back. Now pull up your legs until
they're in   a  90 degree corner, so the lower part of your legs is
about level with the floor (this is an important difference with
standard sit-ups).
Now put your hands behind your ears (so not behind your neck, your not
supposed to pull yourself up with your arms, but with your abs!). If
you still get pain in your neck, cross your arms over your chest)
Now start the sit-ups: pull up far enough to take your shoulders off
the floor. Your nose doesn't have to touch your knees, just up far
enough to feel your abs tighten well.

2.  An other useful exercise is a variation on the previous one.  
Here we go: lay down on the floor again, but this time put your legs
straight up, so they point to the sky. It's important they're as
straight upwards as possible. Now try to touch your ankles, or your
shoes if you're wearing any.  when you're up, hold your position for
1-2 seconds.

3. Another sit-up, but this time diagonally (I hope i'm spelling
everything right btw, i'm Dutch).
Here you go: lay down in the sit-up position again, so with your legs
pulled up 90 degrees. Place your hands behind your ears. Now comes the
new trick: pull up, but immediately start turning your upper body,
left elbow to right knee and vice versa (as mentioned in the previous
comment). It's important to keep your elbows spread, so don't pull
them together. You should feel this in the abs on both sides of your
stomach.

You have different types of abs, and with these three exercises you'll
train all of them. It should be pretty effective.

Try to build up the exercises: if your abs are untrained, start with
fifteen times for every exercise, with a couple minutes rest between
the exercises. After a few weeks, build up to 20 times, and so on.

If you have any questions, post them and if I see them I'll try and answer them. 
Good luck!!!

Let me add in the most important addition to the above
comments...cardiovascular exercise and a healthy diet.  All the situps
in the world won't give you a 6-pack if you're carrying too much body
fat around your stomach.  It's more a function of how visible the
muscles are than how strong/big they are.  (That's why all the
contestants on Survivor are ripped at the end of the show...not
because they do crunches all day).
Good luck.
js