Mechagnomes are small gnome-like clockwork creatures who are found in Northrend. They have a strong connection to the titans, although it's unknown exactly what their original purpose and reason for creation is. Their bodies consists of metallic plates and screws, marked by runic tracks of light. Some of them have mustaches and similar hair-fragments on their heads, also made out of metal. The sounds they make resemble the voices of normal gnomes, just in a "metallic" fashion.
Mechagnomes come in various sizes, but are usually at the size of a normal gnome.
The first mechagnomes, if not all, were created by a titan keeper called "The Grand Architect".
Whether The Grand Architect was a titan or not (see Speculation below), the mechagnomes were apparently also made by the Vanir-titans as one of the races intended to help the titans shape the underground and deep regions of the world.
Mechagnomes are immortal. This does not mean they are invulnerable, just that they can't die from aging. They can, however, malfunction. When they do, they require someone else to repair them.
Matching their appearance, mechangnomes act very robot-like. Their behavior is marked by a pragmatic, objective and sometimes commanding way of thinking, and they seem to have a habit of meticulously analyzing their surroundings. While this trait often may seem odd to the mortal races, it often makes it very easy to understand mechagnomes and pick up the information they give.
When performing an action or a choice, mechagnomes often utter their reasoning out loud. From what they are saying, the minds of mechagnomes are clearly much similar to a computer-program, processing variables and executing commands.
Apparently, mechagnomes have a connection to what could be "titan databanks" - they can sometimes be observed "accessing" these databanks in order to retrieve information they need. This can be done either directly through the Databanks or through nothing at all, as seen when Attendant Tock attempts to contact them when analyzing a piece of strange metal.
Mechagnomes possess the convenient ability of going into standby-mode. They often do this after completing or being released from a task. Most likely they will stand in standby-mode until their master gives them a new order, as they have little reason to do anything else. It is unknown how long a mechagnome can remain in this state. Additionally, mechagnomes can be taken apart without being permanently damaged (as long as they are put back together properly).
Amazingly, normal gnomes can be reverted back into being mechagnomes, despite those gnomes having been flesh for generations. This is not done by merely adding a supplementary spell on normal gnomes in order to transform them, but oppositely by injecting them with something that removes the Curse of Flesh. This means that all normal gnomes are actually still mechagnomes "below" the curse. This undoubtedly proves that normal gnomes are connected to mechagnomes and that mechagnomes are the ancestors of normal gnomes.
Likewise, gnomes who have been turned into mechagnomes may be reverted back to their original fleshy form by injecting them with something that 'reverses the reversion' of the curse. The player has to revert several newly "mechanized" gnomes back to their former self in the quest Re-Cursive (Alliance). After being mechagnomes, most of these gnomes were apparently not fond of having been so. Then again, some of the gnomes felt that being mechanized had "perfected" them.
Engineers can learn how to craft their own mechagnomes. One model is from a quest in the Storm Peaks where the engineer acquires a schematic left behind by Mimir (see SCRAP-E below). The other one is Jeeves (also see below) which is learned from a schematic randomly found when "skinning" mechanical creatures in Northrend.
Gearmaster Mechazod, a mechagnome who has the ability to reverse normal gnomes into mechangomes.
Likewise, The Grand Architect might also be Mimiron.
Since gnomes derives from mechagnomes, their obsession and expertise with mechanics and technology could be explained as a trait from when gnomes were robots. In other words, the gnomes' fascination for making cold steel and electricity into "living" beings may descend from the mechagnomes' sentience of being such constructs themselves. Then again, the mechagnomes' and later the gnomes' interest for engineering may have been implemented by the titans in order to make mechagnomes effective workers.
While they usually appear to be nearly brain-dead, most mechagnomes seem to have some kind of pseudo-consciousness. In the quest Slaves of the Stormforged where you have to free some mechagnomes from the iron-dwarves, some of the mechagnomes are clearly thankful for being freed ("Thank you, <race>. I will join your struggle against the stormforged"). A few special mechagnomes, like Gearmaster Mechazod, are furthermore quite conscious of their own actions, talking like "normal" gnomes.
Although transforming a normal gnome into a mechagnome (and opposite) somewhat changes the gnome's personality, it seems that they don't lose any of their memories in the process. This assumption is based on Fizzcrank Fullthrottle's tale of how the mechagnomes his crew had been transformed into "recognized" him and the other normal gnomes.
Even though most of mechagnomes that have been reverted back into normal gnomes apparently didn't like their "new" metallic shape, it doesn't necessary mean that being a mechagnome feels bad, just very different from being a flesh-gnome. Otherwise, it would be strange that most original mechagnomes act rather relaxed (when they are not fighting) and affectionate towards their master.
Gearmaster Mechazod was found in the Borean Tundra within a pipe during the process of draining water away from the Geyser Fields. It is unknown how he ended up underground in the first place, but as the traces of the titans are found all over Azeroth, it could imply that there are several other mechagnomes buried around the world.