Seaweed is a major component of beach wrack. It often attracts flies that feed on algae. See image below of flies feeding on a blade of Giant Kelp (Macrocystis).
The flies in turn can be prey for terrestrial predators, e.g., birds and spiders. On 8 March, I noticed a few spiders hunting along the wrack line. I wondered what they might be eating and eventually I observed two of them catch and eat flies. In the photo below you can see the fly's head/eyes between the spider's fuzzy palps.
I hadn't focused on these spiders before, so spent some more time with them and took pictures with the hope that someone might be able to help identify them.
Note the superb camouflage against the grains of sand. The spider is ~7-8 mm long.
View from above.
The spider is looking to the right; note the two large forward-directed eyes, four medium-sized eyes, and two very small eyes between the upper and lower medium-sized eyes (hard to see in the photo).
Most of the spiders were gray/brown with white/cream spots and stripes (Would the specific patterns be useful in identifying individuals?) But one spider was black and silver — A different species? A different gender?
ADDENDUM (12 March 2012)
After searching the Internet, the closest I've come to an identification is a jumping spider (Salticidae), possibly Terralonus californicus. This name is offered with some hesitation, as I don't have any experience identifying spiders, but it's the best I can do so far.