I discovered last night that Dominic Keating who plays Reed on Enterprise has a minor role in an episode of Buffy. Season 3, in fact, called "Helpless".
Unsurprisingly he plays a Watcher.
PS: If someone's mentioned this before and I've forgotten, feel free to whap me. My memory is poor.
PPS: If I mention this again in 6 months, feel free to point me to their entry and I will whap myself soundly.