I found atleast 25 sub-genre's of Heavy Metal music, there must be many, many more.
Black Metal - High pitched vocals, similarities to rap.
Celtic Metal - Music mixing traditional Irish folk music with that of Heavy Metal.
Doom Metal - Downturned tempos and thick, heavy musical sounds. Influenced by Black Sabbath.
Gothic Metal - Flamboyant/Melodramatic, with a dark dress sense, vampiric, devils, bats, etc.
Grindcore - often growling in the background and high pitched screams. High Speed tempo and distorted guitar sounds.
Hair Metal/Glam Metal - traditional hard rock music mixed with Punk Rock, strong emphasis on a certain style of dress, long back combed hair, studs, spandex, etc.
Metalcore - Extreme Metal and Hardcore Punk music. Sepultura and Integrity are good examples.
Nu-Metal - Mixture of Grunge and Hip-Hop
Power Metal - Found in mainland Europe, Brazil and Japan. Lighter music with melodies and keyboard music prominent.
Progressive Metal - Guitar lead sounds with complex and unusual structures.
New Wave of British Heavy Metal - Harsher sounds, elements of Punk and a turned up tempo to traditional British Heavy Metal like Led Zep. Examples of this being Iron Maiden and Saxon.
Thrash Metal - Megadeath, Metallica. Deals with social or political issues, trying put a point across, like Megadeath's song "Peace Sells (But Who's Buying?)
Groove Metal - Hard Rock mixed with dance and hip-hop
Industrial Metal - Ministry, Godflesh, Nine Inch Nails
Pirate Metal - Scottish band Alestorm would be a good example of this, they also did a very good version of the Flower of Scotland!
Post-Metal - distorted guitar, gradual build up of the song, least amount of emphasis on vocals.
Sludge Metal - Originating from Southern states of the U.S.A., quite aggressive music, shouted rather than sung vocals and contrasting tempo's.
Speed Metal - Influenced by Motorhead, and to an extent Iron Maiden. Very fast form of metal, use of sounds that are challenging to create.
Stoner Metal - Slow music, with low guitar music. Seen as quite retro.
Avant Garde Metal - also seen as experimental metal, using unconventional sounds, instruments and structure of songs.
Soft Rock/Adult-Orientated Rock/Arena Rock - Good live performances, with loud audiences participating, use of pyrotechnics, smoke bombs, etc. A lot of focus of chorus and vocal harmonies.
Funk - Red Hot Chili Pepper's is a very good example of this mixing classic heavy metal with african american originated Funk music.
Grunge - classic example would be Nirvana, the sub-genre originates from Seattle, Washington in the 1980's, the idea of heavy distorted electric guitars.