Gaia OnlineThe forum with cool customisable avatars that I hang out at has enabled an invitation system so that members can invite their friends to join. You don't need an inviation to join, of course, but there are bonuses if you do get an invitation: Extra game gold on startup.. I think 200 extra gold (there is also a gold bonus for me, but I am not sure how much and it's not per invitation but rather for every five invites or something like that). Normally you would start with either 400 gold or 450 gold (I don't remember which).
Anyway, how to get invited? Reply with the e-mail address you want to use to join Gaia and I will send an invitation to it. (Comments are screened, so you don't have to worry about other people getting it.) My Gaia username is Kristall Fox.
It's a good time to join, Gaia is, right now, in the middle of celebrating their fourth anniversary (it goes from Feb 23rd to Feb 25th, if I am not mistaken), so there are limited edition game items that are being given out.
And what IS Gaia Online? Well, it started out as a forum and link list (anime based) with cool paper-doll style avatars (which you can buy items for to dress them up), then they started adding features like mini games (word bump, jigsaw puzzles, fishing, slots, and others), and a system that allows your avatar to walk around like in an RP game, with various things to do there (it's called Gaia Towns), they have NPC's (non-playable characters) and plotlines which are detailed in an online manga, they have special events like the anniversary ball, halloween, christmas, new years, easter, etc.. all of which you can get special limited game items. They also added a store a while back where you can purchase real merchandise (clothing, posters, accessoires, etc..)
Along with the forum system (which is a heavily modified variation of PHPBB), they have a Guild System which essentially allows users to own mini-forums to use as fanclubs, etc.. I'm a member of The Gaian Parents Guild, the Gaian Parent's Lounge, the Adult Furries Guild (it's not 'adult' as in rated-X, but adult as in: immaturity and netspeak is not welcome - though considering the founder himself is rather immature... he has 'left' Gaia over the release of a game item and various other issues), an Invader Zim Guild, a Music/Arts/Entertainment Guild, and a few others. Though I must admit that my Gaia activity comes and goes in spurts.
Here are some examples of a Gaia Avatar (girl and boy):
(Those are: me, Rofu, me, and Darkmane.)
Hope to see you there!