A nice collection of cheap DIY greenhouse projects that include plans or tutorials.
1 Basic Wooden Box Greenhouse

2 Countertop Greenhouse

3 Small Poly Tunnel

4 Cold Frame

5 PVC Greenhouse

6 GeoDome Greenhouse

7 Greenhouse Under 100 Dollars

8 Easy and Quick Greehouse

9 Solar-heated Greenhouse

10 Polytunnel Greenhouse

11 Barn Greenhouse

12 Movable Seed Starting Greenhouse

13 50 Dollars Greenhouse

14 CD Spindle Case Propagator

15 Extended Shed Greenhouse

16 Geometrical Shape Greenhouse

17 Old Windows Greenhouse

18 Wooden Frame Greenhouse

19 Recycled Containers Mini Greenhouse

20 Small Greenhouse

21 My Little Window House

22 The Baby Greenhouse

23 Used Windows or Storm Doors Greenhouse

24 Homemade Greenhouse

25 CD Case Greenhouse

26 DIY Greenhouse

27 Quick Greenhouse

28 Mini Greenhouse

29 Simple Greenhouse

30 Easy Pallet Greenhouse

31 Umbrella Greenhouse

32 Soda Pop Greenhouse

33 Hoop Style Greenhouse

34 Fold Down Greenhouse

35 Plastic Bottle Greenhouse