One of the requirements for ultra marathons like the UTMB is a waterproof, breathable, lightweight jacket that will fit into your running pack / vest.
Now, these things are expensive, costing as much as $400 for a top of the range Arcteryx jacket (behold it in its glory below).
The Outdoor Research Foray Jacket fits the bill and at under $130, leaves a muuuuuch smaller dent in your pocket. Spencer got one last year and has used it in rainy tropical conditions in Malaysia as a rain jacket, through sub-freezing temperatures over thermal running shirts, here on PA trails. He likes it because it keeps him dry, packs down small, weighs very little (15 oz) and looks nice. An interesting and unique feature are the side zips that zip from bottom all the way to the pits (like an extreme version of a pit zip) which supposedly assists ventilation in highly physical activities. Recommended!