Online Compression

General use

WeCompress

Compress-Online

Compress2Go

YouCompress

PDF

Only compress

ShrinkPDF

Full editing suite

Adobe Acrobat PDF Online

ILovePDF

PDF.io

General converter site

Online-Convert PDF

CloudConvert PDF

Images

ImageOptim

CompressPNG
has sister sites with other formats

Ezgif WebP
also has other formats

Video

123aps Video Converter

YouTube
You can't upload copyrighted or innapropiate material.