  We construct infinitely many new 1-parameter families of simply connected
complete noncompact G_2-manifolds with controlled geometry at infinity. The
generic member of each family has so-called asymptotically locally conical
(ALC) geometry. However, the nature of the asymptotic geometry changes at two
special parameter values: at one special value we obtain a unique member of
each family with asymptotically conical (AC) geometry; on approach to the other
special parameter value the family of metrics collapses to an AC Calabi-Yau
3-fold. Our infinitely many new diffeomorphism types of AC G_2-manifolds are
particularly noteworthy: previously the three examples constructed by Bryant
and Salamon in 1989 furnished the only known simply connected AC G_2-manifolds.
  We also construct a closely related conically singular G_2 holonomy space:
away from a single isolated conical singularity, where the geometry becomes
asymptotic to the G_2-cone over the standard nearly K\"ahler structure on the
product of a pair of 3-spheres, the metric is smooth and it has ALC geometry at
infinity. We argue that this conically singular ALC G_2-space is the natural
G_2 analogue of the Taub-NUT metric in 4-dimensional hyperKaehler geometry and
that our new AC G_2-metrics are all analogues of the Eguchi-Hanson metric, the
simplest ALE hyperK\"ahler manifold. Like the Taub-NUT and Eguchi-Hanson
metrics, all our examples are cohomogeneity one, i.e. they admit an isometric
Lie group action whose generic orbit has codimension one.
