Freemake Video Converter supports .mts files and is under active development. Free, reliable and has very simple drag and drop functionality.
200+ Input Formats
Free Video Converter accepts any input video file: AVI, MP4, MKV, WMV, MPG, 3GP, 3G2, SWF, FLV, TOD, AVCHD, MOV, DV, RM, QT, TS, MTS, etc. Import music (MP3, AAC, WMA, WAV), and photos (JPG, BMP, PNG,GIF) to turn them into video free and easily! 200+ supported formats
It also has a button that uploads straight to YouTube as part of the conversion process.