The most obvious example would be the Violin.
Others are Harmonium, Guitar, Clarinet, Saxphone, Mandolin.
Here are snipets from an article which can be found at
http://www.musicalnirvana.com/introduction/introduction.asp
"Prasanna on Guitar)."
Clarinet
"Clarinet is a wood-wind instrument of European origin. But it was found suitable for playing Carnatic music too. It is played like the Nadaswaram (Nagaswaram). In the 19th century itself Mahadeva Nattuvanar was the first to practise this instrument and introduce it in a dance band in South India. But Clarinet was made popular on the concert stage mainly by 'Clarinet Chakravarthi' A.K.C. Natarajan, who adopted it very well to the needs of carnatic music - raga alaapanas and vocalized techniques.
Saxaphone
Saxophone though, is a recent introduction. The credit goes to Kadri Gopalnath, who began training in nagaswara from an early age. But he was thrilled by the vibrant tone of the saxophone played by the Mysore Palace Bandset and decided to master it. It took 20 years for Kadri to conquer the complex wind instrument. Since the 80s, he has become a regular on Carnatic concert stage, successfully adapting Saxophone to the demands of Gamakas of Carnatic music.
Mandolin
Mandolin was introduced to Carnatic music by the famous child prodigy U.Srinivas. Mandolin is a small instrument which was developed in Italy from existing lutes. It has 4 pairs of strings. It was modified in the 50s to have 4 single strings and the electric block mandolin was born. Mandolin is inadequate to play Carnatic music. Srinivas introduced a fifth string (to the tradition 4 string electric block mandolin), used single strings instead of using pairs of strings. He also tuned the strings differently to get the strings to the right pitch. he devised very ingenious fingering techniques for playing intricate 'gamaka's. All this enabled him to successfully adapt Mandolin to Carnatic music.
Child prodigies always have a magnetic attraction on the concert circuit. Srinivas had become a celebrated concert artist even as he was just entering teenage in the 80s. As with other new instruments, success of mandolin as an established carnatic instrument can only be gauged in the coming decades."