And also they're described as follows: "They behave with cold rationality and have a no-nonsense attitude toward life." Which is to say, they're wombats.
But the most noteworthy thing about them, from a mechanical perspective, is how electricity damage heals them. Every time a mechanatrix would take electricity damage, they take no damage, and it heals them instead, 1 point of healing for every 3 points of damage they would have taken.
So how do you get at-will electricity damage? Should be easier than at-will healing, right? Right! But, as it turns out, not much easier.
The obvious notion is to consult reserve feats, those feats from Complete Mage and Complete Champion that give you supernatural abilities as long as you have a spell of a certain level prepared. And, indeed, there is an electricity damage reserve feat: Storm Bolt (CM).
But wait! Storm Bolt gives you a 20-foot line of electricity. Can you include yourself in a line effect? Let's consult the SRD: "A line-shaped spell shoots away from you in a line in the direction you designate. It starts from any corner of your square and extends to the limit of its range or until it strikes a barrier that blocks line of effect."
So, by RAW, no, not really. It starts in any corner of your square, and shoots away from you. Our mechanatrix could get a mage buddy with Storm Bolt, but he couldn't Storm Bolt himself.
The same problem applies to if you're, say, a Dragonfire Adept or Dragon Shaman with lightning breath (plus you're generally explicitly immune to your own breath weapon).
Or, the allegation goes, you could use a persisted or or permanency'd thunderhead (SpC) spell. A little cloud that floats above your head all day, zapping you with tiny lightning bolts for one electricity damage every round, forever.
Except the mechanatrix isn't healed for 1 point every time they accumulate 3 points of electricity damage they would have taken. Every time they take electricity damage, it's divided by 3, they're healed for that much, and the remainder doesn't matter. 1/3 rounds down to 0 -- it's healing, not damage, so the "all attacks deal at least 1 damage" exception to the "always round down" rule doesn't apply. Thunderhead does nothing.
You could persist or permanency a weapon of energy spell, but then wind up with a bunch of your build or a butt-ton of gold invested in metamagic reducers or a single spell, which is hardly worth it for something so trivial as infinite out-of-combat heals.
You could shell out 8,301gp for a +1 Shock Whip. As long as you're wearing a bit of armor, you can whip yourself all day and only the electricity damage will go through. Expensive, weird, and a little kinky. ...I shall expend no more words on this notion.
You could take a level of electrokineticist. Kind of a lackluster class. It requires a powerpoint reserve but doesn't advance manifesting, so it's a trap for actual manifesters, and you should just use Wild Talent to qualify. But the class features aren't even great for a non-manifester.
Plus, most of the x-kineticist's class abilities specify things like "she takes no damage from a x lash she creates" or xs "engulf one of the pyrokineticist’s hands (but do her no harm)", so you'd have to work out whether an electrokineticist mechanatrix can deliberately target themselves for the full effect of their powers.
An entire class level, just for this ability? There's gotta be something better.
Well, there's something that, by RAW, does work way better: consult Magic of Incarnum, and take the Shape Soulmeld (lightning gauntlets) feat. Can't wear magic gloves, but 1d6 elec damage at will as a touch attack (and it is well known that it is possible to touch oneself).
But... incarnum is (subjectively) kind of lame. It's one of those things, along with Tome of Magic* and Tome of Battle: Book of the Nine Swords, that feels to me too slick and soulless and 4th-edition-y, and which only annoying optimizers ever tend to actually open. There's gotta be something more aesthetically pleasing. More... interesting.
*Binders can allegedly bind Focalor to achieve some form of at-will electricity damage, but honestly just typing this sentence has used up 100% of my ability to give a crap about ToM for the day, and I couldn't possibly find it in my heart to double-check whether this would actually even work.
So here's a more interesting idea: shocker lizard.
You could maybe get a domesticated one for money, but you could much more reliably get one by being a 5th level arcane spellcaster and taking Improved Familiar. (You may also consider trading away your regular familiar for an alternate class feature or the Forlorn flaw (Dragon #333), because the Obtain Familiar (CA) feat is better: it makes prestige classes progress your familiar.) Or you could be a ranger or druid and take Monstrous Animal Companion (Dragon #326).
Anyway, shocker lizard. Your eye might be drawn to its Stunning Shock ability, but alas! It won't work. "An electrical shock" sounds promising, but "this attack deals 2d8 points of nonlethal damage". There's no such thing as nonlethal electricity damage. Nonlethal is its own thing. A point of damage cannot be both nonlethal and electricity, it is either one or the other. The shocker lizard is quite clear: it's fluffed as electricity, but it's actually just nonlethal. (A particularly generous DM might rule that the shocker's nonlethal shock will cure any nonlethal damage the mechanatrix may have taken, but we oughtn't rely on the generosity of the DM.)
No, what we need is Lethal Shock, because that's actually electricity damage. But wait! You need two shocker lizards for that!
Are you seeing what I'm getting at yet?
What I'm getting at is this: Mechanatrixes have tamed shocker lizards, and use them in war and daily life. Mechanatrix society is ruled by a cadre of arcane casters, all with shocker lizard familiars. Mechanatrix adventuring and war parties always include at least two shocker lizards, usually more, with at least one usually being the familiar or animal companion of one of the party's casters.
Next time you use mechanatrixes as a DM, team them up with some shocker lizards. The lizards have been trained to use their lethal shock every round, and the mechanatrixes and lizards all stay within 20ft of one another. (Maybe bump the total ECR of the encounter up by one or so, because synergy.)
As for me, because I'm not currently DMing a game: next time I'm making new characters (at ECL6 or more) at the same time as somebody else (either because it's a new campaign, or because two characters died at the same time), I'm going to try to convince them to make a pair of wandering mechanatrix adventurers with shocker lizard familiars/companions. ("Hey, you feel like making a bard, beguiler, dread necromancer, druid, duskblade, hexblade, ranger, sorcerer, spellthief, wizard, warmage, or wu jen?")
EDIT: Alternately, you could just take Extra Familiar (Dragon #280), which negates the requirement that you have a buddy with the same plan, but which makes it a bit less interesting. On the other hand, every extra shocker lizard adds another 2d8 electricity damage each round (that's extra damage to anything not immune to electricity and an extra ≈3 points of healing for each mechanatrix), so you would certainly benefit from everybody involved having Extra Familiar (at most 6 lizards can contribute to any one lethal shock, but even if you have 7 lizards, you can just have one 5-lizard shock and one 2-lizard shock each round, paying only a slight save DC cost, because the save DC is also a function of the number of lizards involved. Or there's the option to just have more than 6 lizards contribute to a single shock: it would still only be 12d8 damage, but there's no cap on the save DC).