  Accurate photometry with HST/ACS shows that the main sequence of the globular
cluster NGC 2808 splits into three separate branches. The three MS branches may
be associated with complexities of the cluster's horizontal branch and of its
abundance distribution. We attribute the MS branches to successive rounds of
star formation, with different helium abundances; we discuss possible sources
of helium enrichment. Some other massive globulars also appear to have complex
populations; we compare them with NGC 2808.
