I also like the AMK survival bivvies.

For casual day hiking, I carry the Emergency Bivvy in the bottom of my pack. Very lightweight, compact, and only about $17. Probably only a one time use item, but it will keep the rain and wind off to enable you to get through the night.

For more serious adventures, and in my SAR pack, I carry a Escape Bivvy . It is slightly heavier and bulkier than the Emergency Bivvy, but still quite reasonable size/weight for a pack. It is breathable, costs about $50, and is sturdy enough to be used multiple times. It is orange, for high visibility, which is desirable in an emergency situation. However, they also make it in OD Green for those who want to be "Tacticool".