There are literally hundreds of sites to meet other players and talk about Halo. Here are a few of the more popular sites focused on Halo, machinima, and other resources.
Bungie.net
The home and community site of Bungie Studios, the developers that made Halo. Here you can find out about updates with Xbox Live and Halo 3, link your gamertag, view your matchmaking stats, and even see which friends are online. Forums and help sections are here so you can ask other players and employees if you have a problem or a complaint, or want to go really in depth with the Halo backstory. Recent updates have the site a great place to visit.
Halo.Bungie.Org
This an extremely resourceful and probably the best Halo site on the web. Focused mainly on news topics in the Halo world, the HBO team also has extensive resources including fan fiction, the Halo backstory, cut scenes from the games, game dialogue, and loads of other incredible stuff! Check it once a day for a good dosage of Halo.
Halogrid.com
This is the home of HollywoodHalo Productions. They make amazing movies, spoofs, and montages. More notable, there is a great forum with a tight-knit community where you can chat, post movies, discuss topics in the Halo community, and even recruit actors and filmers for your own movies.
RedvsBlue.com
This is the site of RoosterTeeth Productions, one of the original Halo machinima groups. They focus mainly on comedy movies, ranging from the use of Halo, F.E.A.R., and The Sims 2.
HighImpactHalo.org
The trick and glitch site of everything Halo. Learn about every little glitch, secret, and easter egg in all of the Halo games, and even some you don't want to know about! Most gltiches have tutorials or videos attached so you can try them out yourself.