For all those ideas that don't fit any particular age group.  I'll start it off with a few links we've run across.

Official Crayola Site
Over 1,000 craft ideas All ages - free coloring pages, crafts, lesson plans, games, and more

Kids Activities with Printables
Activity TV. Three levels- Beginner, Intermediate, and advanced

Kids activities for children, fun things to do with your child (Savvy Source)
Savvysource.com Encycopedia of activities indexed by age and reviewed by users

Family and Kids Crafts (Howstuffworks)
Find craft ideas by material, theme, or difficulty level

The ChildFun Family Website
Activities and crafts for all ages- not indexed by age, though. Several activities under each theme.

Cooking with children (BBC)
Article from BBC with recipes and good tips on how to handle children in the kitchen

Geekdad - Wired Blogs
Not dedicated to activities per se, but has a lot of interesting ideas not found elsewhere (papercraft Star Wars figures, etc.)

Great rainy day activities to do with your children - Childrens Activities Article on Helium
29 articles from helium on rainy day activities

Kids Games (Centerofweb)
Index of over 250 traditional games for kids- unfortunately, only indexed alphabetically

Kids crafts - free childrens activities and gifts for kids (creativeactivitiesathome).
Creative kids at home lots of activities and creative activities, including a section for younger kids.

kid stuff! - Easy craft activities and projects for kids (Makestuff)
Make-stuff.com some activities have video clips. annoying advertisig on the site, though. Can be all ages, with parental help. i.e., "potato prints"- parent can cut potatoe with design, and child can stamp print.

Kids Games
Website of kids games, indexed by category and alphabetically