Bubble Counter for CO2
This newsgroup discussion offers a unique and inexpensive idea for a CO2 bubble counter.
Cheap CO2 Controller
The title says it all ... anyone can afford the $15 price tag on this dyi CO2 controller.
Do-It-Yourself Carbon Dioxide Injection
Derek Heath and Len Trigg describe how to make a CO2 injector for your planted aquarium, in this article for Net Pets.