http://slaverats.com - Rob Thomas's official site. It has a lot of interesting stuff for VM and his other work.
http://watchveronicamars.net - A website for saving VM
http://community.livejournal.com/cloud_watchers - the site doing lots of campaigns including the DVD drive and the plane to save VM
Mars Investigations -The (In)Complete Guide to Veronica Mars with character bios, ep guides, cast and crew interviews, music guides, a sort of VM encyclopedia, media mentions, and more. Really useful to help new fans catch up and fun for old fans too. This is the most obssesive site I've ever seen. It's really amazing.
Veroncia Mars Caps - Good source of good quality screencaps from every ep and it's not shipper/character biased like some sites and has kept up with the episodes more than some other sites that don't have the bias.
Television Without Pity - Television Without Pity is a site with hilarious recaps of tv shows. I've linked to the VM section. They can be critical and loving of a show.
http://www.livejournal.com/community/vmrandom/ - It's for VM spam. If it has to do with Veroinca Mars and it's not an advertisment you can post it. There is some hilarious stuff there.
http://livejournal.com/community/fic_from_mars/ - fanfic
http://www.livejournal.com/community/tritons/ - any fanfic that's not shipping Logan/Veronica
http://www.livejournal.com/community/vmfanficutopia/ - A place for VM fic challenges that try to focus on all characters.
http://www.livejournal.com/community/rack_of_lamb/ - A community for Sherrif Lamb. Michael Muhney (the actor that plays Lamb) posts here.
http://community.livejournal.com/weevers_unite/ - A weever site