We study various models of dark matter and dark energy. We first examine the implications of the assumption that black holes act as dark matter. Assuming dark matter in galactic halos is composed solely of black holes, and using observational constraints, we calculate the number of halo black holes and the total entropy due to them. We then study the prospect of dark energy with a non-constant density. We analyze several parameterizations of dark energy density from the literature and one of our own, in particular focusing on the value of redshift at which cosmic acceleration due to dark energy begins. In considering the properties of dark energy densities that monotonically increase over time, we present two new categorizations of dark energy models that we dub little rip and pseudo-rip models, and both avoid future singularities in the cosmic scale factor. The dark energy density of a little rip model continually increases for all future time, and a pseudo-rip model's dark energy density asymptotically approaches a maximum value. These two types of models, big rip models, and models that have constant dark energy densities comprise all categories of dark energy density with monotonic growth in the future. A little rip leads to the dissociation of all bound structures in the universe, and a pseudo-rip occurs when all bound structures at or below a certain threshold dissociate. We present explicit parameterizations of the little rip and pseudo-rip models that fit supernova data well, and we calculate the times at which particular bound structures rip apart. In looking at different applications of these models, we show that coupling between dark matter and dark energy with an equation of state for a little rip can change the usual evolution of a little rip model into an asymptotic de Sitter expansion. We also give conditions on minimally coupled phantom scalar field models and scalar-tensor models that indicate whether or not they lead to a little rip or a pseudo-rip.