(Last Updated: 04-July-2008)

Ok so I guess it's about time I sorted out what is what here. So here is my attempt that others might find useful.

Software:

Opensimulator (aka OpenSim) is the open-source sim software itself currently at version 0.5 that can run standalone on your PC or connect to a larger grid on the internet. It has everything needed to connect a SL compatible client to it.

realXtend have developed a brand new viewer (REX) that works with the OpenSim server software, and offers incredible new capabilities - the ones people are freaking out over (HTML on a prim, mesh models, real time shadows, etc). When you use the realXtend viewer to build, there is a new tab that has properties you can set to activate the features (the same way, for example, you turn on flexiprims). For example, to use a mesh, you create a single prim, and check a box to turn on the REX properties. One of those properties is the name of the mesh to use from your inventory. Another properties is if the object casts shadows, etc. Another example is you can right click on a texture in your inventory and set it's media URL, and then that texture when used on a prim will display external content - web sites, external images, etc.

They have also contributed code to the OpenSim project to support the features in the new viewer on the server side, and are actively involved in development of new features. The RealXtend viewer will work with any grid using the latest OpenSim software, and dramatically changes what is possible. It will also work with SL, but the new features are not supported in the SL grid.
(Thanks Cristiano for this addition)

Grids:

OSGrid
Users: 3,197
Regions: 372
Rent Managed Regions: No, However see thread http://www.sluniverse.com/php/vb/oth...tml#post289506 ($US15+/month)
Connect Your Region: Yes (no fee)
Economy: Unknown
Notes:
No TOS found.
osgrid.org * View topic - OSGrid Homesteading
"OSGrid is an organization that provides potential region operators a place to hitch up their regions alongside other regions (even if not necesarily geographically adjacent). For our purposes, this means that the regions on a grid share at minimum User Authentication and Region Authentication services, and potentially Asset and Inventory backend services as well, depending on how the region server in question is configured."

OpenLifeGrid
Avatars: 25,084
Regions: 220
Rent Managed Regions: Yes ($US30 Set Up, $US59 per month)
Connect Your Region: No
Economy: No currency yet
Notes:
Foundation Residents / Virtual Land
"Activities Considered unlawful or illegal including harmful, abusive or 'griefing' to the community are not tolerated and may result in your account being suspended and/or cancelled without a refund. This includes Sexually Explicit and Explicit Violent content, public display and content development must be mindful of this environment supportive of up to only an M 15 environment, and not exceed this."

DeepGrid
Users: 2,055
Regions: 752
Rent Managed Regions: Yes ($unknown)
Connect Your Region: Yes (no fee)
Economy: Unknown
Notes:
DeepGrid - OpenSimulator Grid
"By making Content available, you represent and warrant that: [....] the Content is not obscene, libelous or defamatory, hateful or racially or ethnically objectionable, and does not violate the privacy or publicity rights of any third party [....]"
Recognises IP rights and DMCA

Central Grid
Avatars: 1938
Regions: 321
Rent Managed Regions: Yes ($US94/month for 1-6 regions, larger options also)
Connect Your Region: Yes ($US395+ fee, $US30 per month)
Economy: Unknown
Notes: See thread http://www.sluniverse.com/php/vb/oth...-any-info.html
"We can allow other smaller grids to connect to The Central Grid and utilize our central servers that handle things like assets, inventory, user management and other needed grid resources. If a grid operator was to deploy a server with any amount of regions, they could easily connect the regions to our central grid platform."
(Thanks Quiplash)


Also see: OpenSim:Grids - OpenSim