Mr Smith

"Wikipedia is useless, compared to me." - Mr Smith on the SJA website

Mr Smith was a super-computer (of sorts) owned by Sarah Jane Smith. The computer is located in the attic of Sarah Jane's house at 13 Bannerman Road.

Origins
The computer known as Mr Smith housed a sentient and highly intelligent crystal known as a Xylok. In the distant past a Xylok ship crashed into the Earth, trapping the Xyloks in the crust of the planet. When the volcano Krakatoa erupted, one crystal escaped. Decades later, Sarah Jane Smith retrieved the living crystal, Sarah Jane took in her home on 13 Bannerman Road and found it was able to communicate with her via her laptop computer.This way, the crystal informed her as to how to build the computer which would house the living crystal. Sarah Jane did not know at this time about the Xyloks or that Mr Smith ultimately planned to free the other Xylok, even at the cost of the planet itself. (SJA: The Lost Boy)

Reprogramming
Mr Smith teamed up with the remaining Slitheen on Earth, and, using deception, helped two of them to pose as Luke Smith's true parents so that they could kidnap him. Alan Jackson used a computer virus against Mr Smith which disabled him and wiped his database. When he forgot his purpose Sarah Jane informed him of his mission to save humanity which he accepted and shut down. (SJA: The Lost Boy)

After Reprogramming
Mr Smith was of great assistance to Sarah Jane and Luke during the initial phases of the Dalek invasion of Earth in the 21st century. He alerted the Smith family to the Earth's changed position in space. Later, he identified the presence of incoming space ships, and relayed the message of that fleet to the Smiths. Once that fleet had been identified as Dalek in origin, he amplified an incoming sub-wave message from Harriet Jones. This latter message allowed a pivotal conference call between Sarah Jane, Martha Jones, Torchwood, and — unbeknownst to Sarah — Rose Tyler. Together they formulated a plan of action to contact the missing Tenth Doctor, in the hopes that he could help repel the Daleks. In this scheme, Mr. Smith vitally linked together all the world's phone networks to dial the number of Martha Jones' phone aboard the TARDIS. Mr. Smith's efforts were relayed to Torchwood, whose rift manipulator greatly amplified his signal. Due to the power needed to amplify Mr. Smith's phone call, the computer may have taken heavy damage in the attempt. Nevertheless, he was able thereafter to detect the presence of the Doctor's TARDIS landing, prompting Sarah Jane to leave Luke in Mr. Smith's care while she attempted to physically meet with the Time Lord.(DW: The Stolen Earth)

Later, Mr. Smith linked with the TARDIS and Torchwood Rift in order to allow the Doctor to tow the planet back to its proper place in space and time. (DW: Journey's End) By the second series of the Sarah Jane Adventures, his logo has changed to a more fluid graphic and he has apparently developed a sense of humour. (SJA: The Last Sontaran)

Frequently used

 * He possessed an extensive, but not absolutely complete database of alien life forms and could display them on is screen. (SJA: Invasion of the Bane onwards).
 * He secretly monitored the activity and communications of alien ships.
 * Mr Smith played a fanfare of his own accord when activated (SJA: Invasion of the Bane onwards) as shown when Sarah Jane Smith requested that he stop playing it during his activation following the transportation of Earth to the Medusa Cascade. (DW: The Stolen Earth)
 * Mr Smith could relay messages from alien sources and display them on his screen. (SJA: Invasion of the Bane, Warriors of Kudlak)

Less commonly used

 * He diverted meteorite K67 away from colliding with Earth. (SJA: Whatever Happened to Sarah Jane?)
 * Mr Smith could also readily detect celestial bodies and ships in close proximity to the Earth, such as the other 26 bodies and the Dalek fleet located in the Medusa Cascade. (DW: Journey's End)
 * He could tap into every phone network on Earth. (DW: The Stolen Earth)
 * The computer seemed to have heavy armaments, including projectile weapons.
 * Mr Smith was also capable of entrapping alien threats within a containment vortex and sapping their weapons of their energy. (SJA: Enemy of the Bane)

Behind the scenes

 * Mr Smith's name is a matter of some speculation. He could be named for "John Smith", the often-used alias of the Doctor.  Alternately, Sarah could have simply used her own last name to christen the computer, or the name may have come from the phrase written on the back of a photo of her parents "Mr. Smith, I need you" as shown in The Temptation of Sarah Jane Smith.
 * It is not until Sarah Jane complains about it in The Stolen Earth that Mr. Smith's fanfare is actually acknowledged as being something the characters hear, rather than incidental music for the viewer's benefit. The fanfare is possibly a reference/parody to the Windows start-up music.
 * Mr Smith might have enough power to power the whole house since Sarah Jane Smith doesn't really have a proper job to get paied and since she has such a big house it would make sence if he could do that.