UPDATE: Since writing this blog post Blizzard have disabled the below functionality (icons+colours)

Diablo 3, like World of Warcraft, allows the use of colors in general chat. You can use some pre-defined color names, eg: {c_red}text{/c_red} (to type in red), but you can also enter aRGB values like so:

{c:FFFF00FF}PINK!!! PINK!!! PINK!!! {/c}
{c:FFFF8C00}DARK ORANGE!!! DARK ORANGE!!! DARK ORANGE!!!{/c}
{c:FFD2B48C}TAN!!! TAN!!! TAN!!!{/c}
{c:FF000000}BLACK!!! BLACK!!! BLACK!!! {/c}

Producing:

There are also pre-defined colors that match up to in-game tool tips and item colors:

{c_magic}text{/c_magic}
{c_inferior}text{/c_inferior}
{c_rare}text{/c_rare}
{c_epic}text{/c_epic}
{c_legendary}text{/c_legendary}
{c_questlog}text{/c_questlog} (orange-ish)
{c_whisper}text{/c_whisper} (pink-ish)
{c_bonus}text{/c_bonus} (blue-ish)

Things get more interesting with the use of some icons:

{icon:lmb}
{icon:rmb}
{icon:bullet}
{icon:audio}
{icon:platinum}
{icon:gold}

For example:

Finally, there's also an "{f_alpha_tiny}" tag used for making your text slightly smaller :)

These are simply typed in to the chat window - no hacks, 3rd party apps, or anything like that.

And yes, that is a "platinum" coin icon being used there... possibly a new, higher-value, currency coming in an expansion? We'll have to wait and see!

(I discovered these through data mining, but I'm sure others either have - or will soon - find them too. Unfortunately that means we can expect highly colorful and creative, but annoying, chat spam)

UPDATE: Since writing this blog post Blizzard have disabled the below functionality (icons+colours)