  Bayesian atmospheric retrieval tools can place constraints on the properties
of brown dwarfs and hot Jupiters atmospheres. To fully exploit these methods,
high signal-to-noise spectral libraries with well-understood uncertainties are
essential. We present a high signal-to-noise spectral library (1.10-1.69
microns) of the thermal emission of 76 brown dwarfs and hot Jupiters. All our
spectra have been acquired with the Hubble Space Telescope's Wide Field Camera
3 instrument and its G141 grism. The near-infrared spectral types of these
objects range from L4 to Y1. Eight of our targets have estimated masses below
the deuterium-burning limit. We analyze the database to identify peculiar
objects and/or multiple systems, concluding that this sample includes two
very-low-surface-gravity objects and five intermediate-surface-gravity objects.
In addition, spectral indices designed to search for composite atmosphere brown
dwarfs, indicate that eight objects in our sample are strong candidates to have
such atmospheres. None of these objects are overluminous, thus their composite
atmospheres are unlikely a companion-induced artifact. Five of the eight
confirmed candidates have been reported as photometrically variable, suggesting
that composite atmospheric indices are useful in identifying brown dwarfs with
strongly heterogeneous cloud covers. We compare hot Jupiters and brown dwarfs
in a near-infrared color-magnitude diagram. We confirm that the coldest hot
Jupiters in our sample have spectra similar to mid-L dwarfs, and the hottest
hot Jupiters have spectra similar to those of M-dwarfs. Our sample provides a
uniform dataset of a broad range of ultracool atmospheres, allowing
large-scale, comparative studies, and providing a HST legacy spectral library.
