Older but handy,this dictionary/concordance lets you look up a term ["Man"], read the text to which it is related or in which it occurs ["The manner in which the best men are treated in their own state is so grievous..."], and note the work or source of the quote [Republic, VI, 488].
Credo includes the full text of over 600 different reference works. The Philosophy section has 37 titles ranging from aesthetics to feminist philosophy, ancient greek philosophy, political theory, even propaganda.
Intended as a competitor for the 1998 "Routledge Encyclopedia of Philosophy," and the most recent general philosophy encyclopedia we have. Attempts not just to update its classic 1967 1st. edition, but to enhance coverage of culturally diverse philosophical systems.
"From its inception, the SEP was designed so that each entry is maintained and kept up to date by an expert or group of experts in the field. All entries and substantive updates are refereed by the members of a distinguished Editorial Board before they are made public. Consequently, [it]... maintains academic standards while evolving..."