  We study close encounters involving massive main sequence stars and the
evolution of the exotic products of these encounters as common--envelope
systems or possible hypernova progenitors. We show that parabolic encounters
between low-- and high--mass stars and between two high--mass stars with small
periastrons result in mergers on timescales of a few tens of stellar freefall
times (a few tens of hours). We show that such mergers of unevolved low--mass
stars with evolved high--mass stars result in little mass loss ($\sim0.01$
M$_{\odot}$) and can deliver sufficient fresh hydrogen to the core of the
collision product to allow the collision product to burn for several million
years. We find that grazing encounters enter a common--envelope phase which may
expel the envelope of the merger product. The deposition of energy in the
envelopes of our merger products causes them to swell by factors of $\sim100$.
If these remnants exist in very densely-populated environments
($n\gtrsim10^{7}$ pc$^{-3}$), they will suffer further collisions which may
drive off their envelopes, leaving behind hard binaries. We show that the
products of collisions have cores rotating sufficiently rapidly to make them
candidate hypernova/gamma--ray burst progenitors and that $\sim0.1%$ of massive
stars may suffer collisions, sufficient for such events to contribute
significantly to the observed rates of hypernovae and gamma--ray bursts.
