The simplest definitions are:

PeterCat's Infopage - A must-have for all furs. Contains information and links to conventions, publications, and other furry needs.

alt.lifestyle.furry FAQ - A great source of information defining furriness in many of its forms.

  Furs For Peace

Yerf - for all ages (no adult art)

VCL - contains adult art

Furtopia

FurNation

Rabbit Valley Comics - sells comics, graphic novels, artwork, videos/DVDs, and other furry stuff. Very nice creatures! (NOTE: RV also sells adult/mature furry items not meant for younger audiences. However, the pictures on their website have been altered to be viewable by all.) 

Furbid - sells art, videos, fursuits, commissions, and all other things furry. Like Ebay, but furry. *smiles*

 Vital Ground Foundation

 Defenders Of Wildlife

 World Wildlife Federation

 Furries Against Hunting

www.rathergood.com kittens gone wild and other funny things! NOTE: swear words present on this site and in its contents.

Conventions: listed by month (note: some conventions are especially for furries [note the use of the word "fur" in their names] and others are scifi/fantasy conventions where furries are an accepted part)

• Arisia: www.arisia.org (scifi/fantasy con) Boston, MA; early registration discounts; at the door: $40
• Aussie Gather: ausgather.sommerland.artamir.org (furry con) Sydney, Australia; early registration and at the door $??
• Further Confusion: www.furtherconfusion.org/ (furry con) San Jose, CA (near San Francisco); early registration discounts; at the door $??

• Murrrdi Gras: www.stlf.org/ARF/mg.htm (furry parade and events) St. Louis, MO; individual events approximately $35 total
• Furry Weekend Atlanta: www.furryweekend.com (furry con) Atlanta, GA; early registration discounts; at the door ??

• PawPet Megalex Convention: www.ppmp.info/ (pawpets are animal hand puppets) Orlando, FL; con fees $??
• I-CON: www.iconsf.org/ (scifi/fantasy con) Stoney Brook, NY (Long Island); con fees $??

• Rocket City Fur Meet: narf.wereanimal.net/ Huntsville, AL; at the door $25
• C-ACE (Canadian Anthro & Cartoon Expo): www.c-ace.org/ (furry con) Ottawa, Ontario, Canada; at the door CDN$40

• Ohio Morphicon: www.morphicon.org (furry con) Columbus, OH; early registration $30

• ComicCon International: www.comic-con.org (scifi/fantasy con) San Diego, CA; early registration discounts; at the door $60
• Anthrocon: www.anthrocon.org/ (furry con) Philadelphia, PA; early registration discounts; at the door $45

• Feral!: www.campferal.org/ (furry summer camp/workshops) Ontario, Canada (near Minden and Carnarvon); refunds given for referrals of other campers; camp fees CDN$279 + CDN$30 for bus from Toronto to Feral!
• Eurofurence: www.eurofurence.org/EF9 (furry con) Samopše, Czech Republic; 105 EURO (includes con, room, food, linens except towels)
• Mephit FurMeet: www.mephitfurmeet.org/ (furry con) Memphis, IL; early registration discounts; at the door $40

• ConiFur: www.conifur.org/ (furry con) Tacoma, WA; early registration discounts; at the door $45

• FurFright: www.furfright.org (furry con) Hartford, CT; early registration discounts; at the door $15

• MidWest FurFest: www.furfest.org/ (furry con) Woodfield, IL (outside Chicago); early registration discounts; at the door $40
• LosCon: www.loscon.org/loscon/30 (scifi/fantasy con) Burbank, CA (outside Los Angeles); early registration discounts; at the door $45

• PhilCon: www.philcon.org/ (scifi/fantasy con) Philadelphia, PA; early registration discounts; at the door $??

• BritFur: www.britfur.co.uk/ (furry con); date and place still being determined (February). Check out their website! (the link seems to redirect you now to another website but I could not find reference to the convention itself)