You should already have ffmpeg if you've got JDownloader installed, for example:
Appdata > Local > JDownloader (v)2.0 > tools > Windows > ffmpeg > x64



ffmpeg -i "INPUT.mp4" -vf scale=1280:-2:flags=lanczos -c:v libvpx -crf 10 -b:v 1M -an "OUTPUT.webm"

@HD ffmpeg -i "INPUT.mp4" -vf scale=1680:-2:flags=lanczos -c:v libvpx -crf 10 -b:v 1M "OUTPUT.webm"

@1920   ffmpeg -i "INPUT.mp4" -vf scale=1920:-2:flags=lanczos -c:v libvpx -crf 10 -b:v 1M -r 24 "OUTPUT.webm"

ffmpeg -i in.avi -fs 100M out.avi

@1920 No Sound   ffmpeg -i "video.mp4" -vf scale=1920:-2:flags=lanczos -c:v libvpx -crf 10 -b:v 1M -an -r 30 "video.webm"

No audio
ffmpeg -i "test.mp4" -c:v libvpx -crf 10 -b:v 8M -an "test.webm"

With audio
ffmpeg -i "test.mp4" -c:v libvpx -crf 10 -b:v 9M "test.webm"

-c:a libvorbis for better quality audio??  (don't need)
-c tells it to choose a channel, either audio or video, and then you specify the library type
-i is the input
-vf is the videoframe scale
-fs limits the filesize
-lanczos is the rescaling method
-b:v is the bitrate
"-an" means no audio
-ss 00:03:23 -to 00:03:43
-crf 10 is the quality for constant quality mode
-r is the framerate

Change framerate
ffmpeg -i "INPUT.mp4" -vf scale=1280:-2:flags=lanczos -c:v libvpx -crf 10 -b:v 1M -an -r 12 "OUTPUT.webm"

**remove -an for audio**
96DPI is the default and still works with videos...  But you can enter a dimension directly in pixels...

Generate a gif from a video file
ffmpeg.exe -i "input.webm" -r "12" test.gif

Convert an image to ascii art
Masking layer the text to reveal the image

Cutting out video to make webms
ffmpeg -i in.webm -vcodec libvpx -b:v 1M -q:v 10 -ss 00:03:23 -to 00:03:43 out3_1.webm
ffmpeg -i input.mkv -c:a libcorbis -b:v 10M -q:v 10 -ss TIMECODE -to TIMECODE output.webm

GIF to Webm
ffmpeg -i "Input.gif" -crf 10 -b:v 10M -r 60 "Output.webm"

Audio Re-encoding
ffmpeg -i "Input.mkv" -acodec aac -ar 44100 -b:a 192k -vcodec copy "Output.mkv"

Webm to MP4 for when Powerpoint fucks up
ffmpeg -i video.webm -movflags faststart -profile:v high -level 4.2 video.mp4

ffmpeg -i "video.mp4" -c:v libvpx -crf 10 -b:v 8M -r 30 "video.webm"
ffmpeg -i "video.mp4" -crf 10 -b:v 8M "video.webm"


Take a still image and add audio to it
Needs -shortest or it loops infinitely.
Leave out the -shortest if the audio is longer than the video!

****ffmpeg -r 1 -loop 1 -i "Input image.png" -i "Input audio.wav" -acodec copy -r 1 -shortest Output.mp4

ffmpeg -r 1 -loop 1 -i IMAGE.jpg -i AUDIO.wav -acodec copy -r 1 -shortest OUTPUT.mp4

ffmpeg -loop 1 -i image.jpg -i audio.wav -c:v libx264 -tune stillimage -c:a aac -b:a 192k -pix_fmt yuv420p -shortest out.mp4

ffmpeg -loop 1 -y -i "Input.png" -i "Input audio.wav" -shortest -acodec copy -vcodec mjpeg result.mp4

Make sure image does not have any transparency
ffmpeg -loop 1 -i image.png -i audio.wav -c:v libvpx -pix_fmt yuva420p -c:a libvorbis -b:a 128k -shortest out.webm
ffmpeg -loop 1 -i image.png -i audio.wav -c:v libx264 -tune stillimage -c:a aac -b:a 192k -shortest out.mp4
ffmpeg -loop 1 -y -i "image.png" -i audio.wav -shortest -acodec copy -vcodec mjpeg result.mp4 (?)

-pix_fmt yuva420p -metadata:s:v:0 alpha_mode="1"

Extract audio from webm
ffmpeg -i audio.webm -vn -ab 128k -ar 44100 -y audio.ogg
ffmpeg -i audio.mp4 -vn -ab 128k -ar 44100 -y audio.ogg


No need to extract, you can downmix two audio channels and keep video channel and generate a new file in one command.

ffmpeg -i input.mp4 -c:v copy -ac 1 mono-output.mp4