Combine multiple AVIs:

MENCODER:

  mencoder -oac copy -ovc copy -o output.avi input1.avi input2.avi

FFmpeg:

  From the FFmpeg Wiki article on how to concatenate (join, merge) media files:

  Create a file "mylist.txt" with all the files you want to have concatenated in the following form ( Lines starting with a dash are ignored ) :

  # this is a comment
  file '/path/to/file1'
  file '/path/to/file2'
  file '/path/to/file3'

  Note that these can be either relative or absolute paths. Then you can encode your files with:

  ffmpeg -f concat -i mylist.txt -c copy output