The Ambitions expansion pack introduces a new playable character, the SimBot.  There is some speculation that the SimBot is a precursor to Servo, the beloved robot from The Sims 2.  (The Sims 3 is technically a prequel to The Sims 2, and takes place about 50 years beforehand.)

There are two ways to get a SimBot.  If your Sim accumulates 40,000 Lifetime Happiness points, they can cash them in on the "My Best Friend" reward.  A SimBot will be sent out to your house, and will arrive by taxi.  Its gender and name will be pre-determined by the game.

The other way is quite a bit more complicated!  If you have an Inventor Sim who invents all the items there are to invent, they will be offered a sort of treasure hunt by the game.  First they will get an opportunity that involves bringing Palladium ore to the Science Lab.  This leads to an opportunity where you bring in 10 Life Fruits, then one heart-cut diamond, then 100 units of scrap.  After that, you are given the ability to create a SimBot at the inventor's bench.

(After you have built your first SimBot, you can choose the gender of any future SimBots that you create.)

Much like Servo from The Sims 2, SimBots are playable characters which are a lot like Sims in an unusual costume that make funny noises.  In other words, a robot version of playable ghosts.  You can control them just like other Sims, they have Needs and Wishes and Favorites and Traits.  All SimBots are Hydrophobic and Brave, and come with skill points in Inventing and Handy.  They can fall in love and Woo Hoo, although they cannot have babies.

My SimBot was named Carissa, and came with a gardening-related Lifetime Wish (grow a perfect specimen of 10 different plant species).  This is a little problematic, because SimBots can be shorted out when exposed to water, for example if you make them take a shower - or if they play with the sprinklers I always install in the garden.  (A Sim with level 7 Handiness can upgrade sprinklers to auto-water, which is very handy for gardening.)  Although one benefit to shorting out is that your Sims can reprogram the SimBot's traits while they are unconscious!

Servo was essentially immortal, but apparently SimBots do eventually die.  I have even seen photos of SimBot ghost!  I wonder if you could resurrect a SimBot as a playable ghost?  The next obvious step would be to turn it into a mummy, of course!

SimBots are definitely an interesting new addition to the family, but I have to admit I was a little disappointed.  They don't seem to give too many advantages over a regular Sim.  I was hoping that the SimBot would be tireless, or immortal, or endlessly interested in cleaning the house.  So far, mine has just puttered around and played with the trampoline a bit, before falling asleep and sawing digital logs. (SimBots snore in 1s and 0s as opposed to the Zs of a regular Sim.)