Scale and crop

ffmpeg -i ... -vf "scale=w:h, crop=w:h:x:y" ...

Extract 1 frame

See https://trac.ffmpeg.org/wiki/Create%20a%20thumbnail%20image%20every%20X%20seconds%20of%20the%20video 

ffmpeg -i input.flv -ss 00:00:14.435 -vframes 1 out.png

Seeking trick

For better seeking speed, use -ss before the -i and after 

ffmpeg -ss 120 -i input.flv -ss 10 -vframes 1 out.png
or 
ffmpeg -ss 130 -i input.flv -ss 0 -vframes 1 out.png

Crossfading example

This came from http://superuser.com/questions/1001039/what-is-an-efficient-way-to-do-a-video-crossfade-with-ffmpeg 

ffmpeg -i fadeoutclip.mp4 -i fadeinclip.mp4 -an
-filter_complex
    [0:v]trim=s=0:e=9,setpts=PTS-STARTPTS[firstclip];
    [1:v]trim=s=1,setpts=PTS-STARTPTS[secondclip];
    [0:v]trim=s=9:e=10,setpts=PTS-STARTPTS[fadeoutsrc];
    [1:v]trim=s=0:e=1,setpts=PTS-STARTPTS[fadeinsrc];
    [fadeinsrc]format=pix_fmts=yuva420p,      
                fade=t=in:st=0:d=1:alpha=1[fadein];
    [fadeoutsrc]format=pix_fmts=yuva420p,
                fade=t=out:st=0:d=1:alpha=1[fadeout];
    [fadein]fifo[fadeinfifo];
    [fadeout]fifo[fadeoutfifo];
    [fadeoutfifo][fadeinfifo]overlay[crossfade];
    [firstclip][crossfade][secondclip]concat=n=3[output]
-map [output] <add in encoding part here>

Tagging