<p>I have a program in which I've lost the C++ source code.  Are there any good C++ decompilers out there?</p>

<p>I've already ran across <a href="http://boomerang.sourceforge.net/" rel="noreferrer">Boomerang</a>.</p>

You can use
IDA Pro
by
Hex-Rays
. You will usually not get good C++ out of a binary unless you compiled in debugging information. Prepare to spend a
lot
of manual labor reversing the code.
If you didn't strip the binaries there is some hope as IDA Pro can produce C-alike code for you to work with. Usually it is very rough though, at least when I used it a couple of years ago.