Where can i get a program to convert AVI to MKV format?

i want to convert my saved movies on my pc to MKV format due to it make a 700mb movie into a 300mb movie. what program can do this? i have 1000's of programs to convert mvk to avi but i want avi to mkv only. I would prefer shareware or freeware if possible.a link to a web site would be helpfull . Again i want to convert AVI to MKV format.