In today's ever shrinking education budgets, finding something free that can substitute for costly materials is always a good thing.
I use virtual labs and simulations in my physics classes often due to lack of lab equipment or safety concerns. My favorite is PhET.
One activity that I know a lot of Biology teachers do is the owl pellet dissection. However, owl pellets are not cheap and they are one use. I just learned about a virtual owl pellet site, KidWings, from Richard Byrne on his blog.
The Virtual Owl Pellet Dissection lets students take apart an owl pellet and explore what the owl ate. The site is Flash based. It was fun working through the simulation.
The site also has other information about birds and is a great resource for science teachers.