Omnitrix,

The Omnitrix is Ben 10's watch that allows him to transform into 10 different alien forms but there can be more aliens locked that Ben can unlock.

First Ten
Magick - A Sourcer from the star Sourcry. Splithack- A Hacker Prime from the planet Commputerr. Mechaval - A Galvanic Mech from the Galvan Mark II.