The Database of Useful Biological Numbers - great stuff, although before using any hard-to-measure number, be sure to know how the measurement was done before you trust the result.

Check these out:

Number of hairs in human eyebrows: 600

Respiratory cost for slow growing gradd: 2.4 mmole ATP/g dry weight * day

ATP requirement for the creation of an E. coli cell: 12-20 billion ATP molecules.

Average diameter of a protein in E. coli: 5 nm

Fraction of total body energy that is used to drive sodium/potassium pumps in the human brain: 10%

Serotonin content in pineapple: 17ug/g

Total nasal epithelial cell surface area in a mouse: 300 mm^2

Lactose content in Reindeer milk: 2.5%

These numbers may not be good for anything, but do you really need to justify knowing the lactose content in Reindeer milk?