The following are interesting sites which provide similar types of data as MAST's High Level Science Products but which concern other wavelength regions, or which are not obtained by NASA missions:
Paranal High-Resolution Optical Atlas of Field Stars
This site (under development) contains optical high resolution specra which can be nicely merged with MAST datasets.
McDonald Observatory high resolution spectral atlas
This site contains fits data for 118 stars of type A7 and later.
Low-dispersion IUE time-series browser
This contains browser images only, and so far only low-dispersion data.