PLEASE DOWNLOAD THE LATEST VERSION.
Version 2.2:
- Updated usages and average stats to reflect DougJustDoug's December stats
- Implemented many more moves
- Now also provides the Pokemon taking the least amount of damage from your moveset
- Implemented Tyranitar's Sand Stream Special Defense Boost
- Implemented Latias and Latios' Soul Dew Special Defense Boost
- Implemented Clamperl's DeepSeaScale Special Defense Boost
- Fixed usages of Pokemon having two abilities
- Fixed maximum allowed Attack and Special Attack stats.
- Fixed damage calculation yet again (it needlessly multiplied the damage by 0.84 twice in version 1.1)
- Added Rotom forms, Origin Giratina and Shaymin Sky Form
- Updated usages and average stats to reflect DougJustDoug's October stats
- Implemented Electivire's Motor Drive
- Fixed damage calculation
- Added Spacial Rend and Rock Smash
- Zipped File
- First release
Note: This Excel sheet was created on Microsoft Excel 2003. It probably won't work on older versions of Excel, though it doesn't hurt to try. It should be able to work on more recent versions, but I haven't tested it personally to know.
It is available here (version 2.2). (Right-click on the link, and choose Save Target As. It is a ZIP file.) Then double-click on the file and you should be presented with something like this:

Let's start twiddling with this, shall we? Suppose we want to check the type coverage of Modest Gengar with 252 EVs in SpA having Shadow Ball, Focus Blast, Thunderbolt and HP Ice in the Standard metagame. We click on the box next to Type 1 and choose Poison, and then on the box next to Type 2 and choose Ghost, signifying Gengar's typing. Like so:

Okay, on to the stats. Click on the box next to the SpA stat and type in '394', which is Gengar's maximum SpA stat, and press ENTER.
On to the moves. Click on the box next to Move 1 and choose 'Shadow Ball'. Like this:

Notice that the Average Damage box immediately updates with the average damage that a Gengar with 394 SpA and Shadow Ball only in its attacking moveset would deal:

Shadow Ball alone would deal an average damage of 51.47% in the Standard metagame, which is pretty good.
Now let's add Focus Blast to the moveset. How much average damage would Gengar deal having Shadow Ball and Focus Blast in its moveset?

Together, they would deal an average of 65.88% of damage.
Finally, we'll add Thunderbolt and HP Ice by choosing them from the boxes next to Move 3 and Move 4 respectively. Our Gengar would be able to deal an average of...

That's very good damage indeed.
Now suppose we want to see if replacing HP Ice by Energy Ball would make Gengar deal more or less damage on average. It's easy; just choose Energy Ball from the box next to Move 4, and...

It seems like Energy Ball has less type coverage than HP Ice on this particular Gengar, since the average damage has been lowered. You can try replacing Energy Ball with other moves instead of with HP Ice. In our example, we'll revert back to the original moveset of Shadow Ball/Focus Blast/Thunderbolt/HP Ice.
Now how would our Gengar fare in the Uber metagame? To answer this question, click on the box next to Metagame, and click on 'Uber'. Like this:

The Excel sheet would immediately tell us the average damage in the uber metagame:

It would deal almost 6% less damage on average in the Uber metagame than in the Standard metagame.
Hope you'll find this Excel sheet useful!