You can use plastic soda bottles to create a hanging garden like Recyclart did.
Or use bottles as a cloche to protect young seedlings from weather and help them grow.
Another style of cloche from House and Gardening Addicts.
I love these clever self watering pots by Desperate Gardner.
And finally, the automatic plant waterer by Practically Living.
I hope these ideas motivate you to reuse some things you already have...I'm off to dig in my recycle bin for inspiration right now!