The protector Terminator known as Cameron, from The Sarah Connor Chronicles, has laughed on a few occasions. The first was right at her introduction in 1x01, when she introduces herself to John and pretends to be human.
Another, the screenshot below, is from 1x02, Gnothi Seauton, around 15 minutes in. After Sarah makes a deal with someone, he comments on how quiet Cameron is and then laughs. Cameron joins in, presumably to try and indicate she's normal, after having been so expressionless.
It looks pretty natural and not forced during the laugh, although the sudden change of expression (both stopping and starting) does look a bit odd: