Readable is the most comprehensive readability tool.

Readable currently uses:

Grade Levels
Flesch-Kincaid
Gunning Fog
Coleman Liau
SMOG
Automated Readability Index
FORCAST
Powers Sumner Kearl
Rix

Graphs
Raygor Readability Graph
Fry Readability Graph

Scores
Flesch Reading Ease
CEFR
IELTS
Spache
Dale-Chall
Lix
Lensear Write

New scores are regularly added.