Summer Glau - Seconded!!
And Lexa Doig from Andromeda.
@Micael Steven Martin
"I look like a Jewish horse/monkey. But I'm pretty OK with it." - Sarah Silverman
This may fall outside the realm of geek crush and more into the growing-up crush category, but I'll go with Winnie Cooper (Danica Mckellar) from The Wonder Years.
On second thought, maybe she should count as a geek crush; she has written 3 math books!