llInstantMessage

void llInstantMessage(key AvatarID, string Text)

IMs Text to the user identified.

Send Text to the user as an instant message.

Parameters

Technical Details

Tell the owner when an object is touched:

default
{
    touch_start( integer total_num )
    {
        llInstantMessage( llGetOwner(), "Someone touched me" );
    }
}

Send an IM to a detected avatar only:

default
{
    touch_start( integer total_num )
    {
        llInstantMessage( llDetectedKey(0), "Hands Off!");
    }
}

Caveats

All object IMs are throttled at a maximum of 2500 per 30 minutes, per owner, per region, in a rolling window. Throttled IMs are dropped.
Messages longer than 1023 bytes will be truncated to 1023 bytes. This can convey 1023 ASCII characters, or fewer (around 512 UTF-8 characters) if non-ASCII characters are present.
If the specified user is logged in, the message will appear in their chat window and will not be logged by the InstantMessage logging facility.
If the specified user is not signed in, messages will be delivered to their email just like a regular instant message, if the user has enabled email for their account.
If messages are sent to the same user by the same object within about 65 seconds, they will be bundled together in a single email.

llRegionSayTo may be a better choice if the target is in the same region as the object sending the message, as it has no built-in delay and can communicate directly with objects, as well as with avatars and their attachments.
Instant Messaging allows communication from an object to an avatar anywhere on the Grid. However, an object cannot receive an Instant Message.
Using llInstantMessage from one or more child scripts will avoid delays in the main script. Child scripts will still be subject to delays, message queue limits, and region throttles.

The throttle is on all IMs from the object owner. It does not disable all IMs in the region, but does disable all IMs from the owner of the object.
The throttle is not per object, but per owner. Splitting the spamming object into multiple objects will not help unless owned by different people.
2500 IMs in 30 minutes will trigger the block.
IMs that are blocked continue to count against the throttle. The IM count must drop below 2500 before any IMs will be delivered.
The IM count of the previous window is used to approximate the rolling window. If it is 20% into the current window, the IM count will be the current count + 80% of the previous count. This allows approximation of a rolling average, but results in a flood of IMs affecting the throttle for double the window length. In practice, the throttle behaves more like 5k in 1hr than 2.5k in 30 minutes.