For TeX primitive commands a rather nice quick reference booklet, by John W. Shipman, is available; it's arranged in the same way as the TeXbook. By contrast, you can view David Bausum's list of TeX primitives alphabetically or arranged by “family”. Either way, the list has a link for each control sequence, that leads you to a detailed description, which includes page references to the TeXbook.
There doesn't seem to be a reference that takes in Plain TeX as well as the primitive commands.
An interesting LaTeX “cheat sheet” is available from CTAN: it's a list of (more or less) everything you 'ought to' remember, for basic LaTeX use. (It's laid out very compactly for printing on N. American 'letter'; printed on ISO A4, using Adobe Acrobat's “shrink to fit”, it strains aged eyes…)
For command organised references to LaTeX, Karl Berry (et al)'s LaTeX reference manual is (to an extent) work in progress, but is generally reliable (source is available on the.archive as well).
Martin Scharrer's “List of internal LaTeX macros” is a help to those aiming to write a class or package.
The reference provided by the Emerson Center of Emory University), LaTeXe help also looks good.
Source: Reference documents