As of January 22, 2018, it is apparent that some online seed generators (third party websites that generate 81 character strings of text) have been compromised, which has led to many stolen balances. In light of this situation, the IOTA community is urging the complete AVOIDANCE of online seed generators. Please do not use a website to generate your seed.
cat /dev/urandom |tr -dc A-Z9|head -c${1:-81}
cat /dev/urandom |LC_ALL=C tr -dc 'A-Z9' | fold -w 81 | head -n 1
The PowerShell method has been demonstrated to be insecure. Do not use it.