There's a lot of different ideas on what languages are related and how. Some are well-received by most linguists, and others are very controversial. There's a wikipedia entry for all suggested language families that Glottolog has rejected. (**EDIT I've seen some errors in there, I'm gonna improve it.)
Ethnologue, MultiTree and Glottolog. Ethnologue is SILs big catalogue, MultiTree is Linguist Lists initiative to make lots of hypotheses of language history accessible and Glottolog is a huge database of bibliographical information by Harald Hammarström, Sebastian Nordhoff, Robert Forkel and Martin Haspelmath of the MPI Society.
Most often I prefer using Glottolog. Glottolog gives a reference for each point of the tree and classification, i.e. you can see which sources has been used to argue a particular relationship or grouping. Glottolog is more "splitting" than Ethnologue, i.e. more prone to not postulate deep groupings. You can read more about that here.
Btw, remember. If you're not sure if what you're looking for has been classified as a languoid, dialect, top-level-family, isolate or some node in between or if you're not sure of the iso 639-3 code, glottocode or the name glottolog has chosen as default, then search glottolog here and not in the languoids "table". Then you'll search all levels and all alternate names.