In a road race- I carry nothing but a little food and my beverage of choice (Accelerade). If the weather is iffy and may go south during the event, I may have a pair of armwarmers. That's about all I take.

If the race is poorly supported, without any support vehicles or a broom wagon- maybe a small cell phone, tucked in a ziplock bag so I could call someone to come get me if I had a mishap.