The Arecibo Legacy Fast ALFA (ALFALFA) survey is an on-going second generation blind extragalactic HI survey exploiting Arecibo's superior sensitivity, angular resolution and digital technology to conduct a census of the local HI universe over a cosmologically significant volume. When complete in 4-5 years, ALFALFA will detect more than 25,000 extragalactic HI line sources out to z~0.06, and its catalog will be especially useful in synergy with wide area surveys conducted at other wavelengths. ALFALFA is detecting HI masses as low as 10**6 solar masses and as high as 10**10.8 solar masses with positional accuracies typically better than 20 arcsec, allowing immediate identification of the most probable optical counterparts to each HI detection. In the region of the Virgo cluster of galaxies, a number of optically dark HI sources have already been found. These all lie in the outskirts of the cluster and could be tidal or "harassment" debris, the result of high speed gravitational encounters. First ALFALFA results already suggest, in agreement with the results of previous, more limited studies, that there does not appear to be a cosmologically significant population of optically dark but HI rich galaxies. ALFALFA promises a wealthy dataset for the exploration of many issues in near-field cosmology and galaxy evolution studies, setting the stage for their extension to higher redshifts with the Square Kilometer Array.

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