spell(1) collects words from the named documents and looks them up in a spelling list. Words that neither occur among nor are derivable (by applying certain inflections, prefixes or suffixes) from words in the spelling list are printed on the standard output.
deroff(1) reads each file in sequence and removes all roff command lines, backslash constructions, macro definitions, eqn constructs (between “.EQ” and “.EN” lines or between delimiters), pic pictures, and table descriptions and writes the remainder to the standard output.
spell(1) and deroff(1) did not appear in freely redistributable BSD releases for licensing reasons. After Caldera relicensed early UNIX releases the 4.4BSD version was added to NetBSD and OpenBSD.
On FreeBSD you can simple type make to compile both commands.
When installing it on FreeBSD via ports or package, it's will be installed as bspell, since it would otherwise conflicts with the aspell-ispell port / pkg.