Is there an explanation for the -ght found in so many words
that seem to have no other connection to each other besides
these three letters? Was there some sort of "contraction" of
words early our languages' development that resulted in these?
I am thinking of words like: might, fought, sight, wrought,
sought, light, etc.

The -gh- represented a back of the throat sound no longer present in the language. Besides the words ending in -ght there are a number ending in -gh. I can't think of any ending in -gh- followed by any other letter but that doesn't mean there aren't any. But, no, it's not the remnant of any contraction.

BTW, is slough pronounced sluff?

Thats how I've always pronounced it.

Wiki says it rhymes with cow.
http://en.wikipedia.org/wiki/Slough

This is the pronunciation heard in the British TV series "The Office", set in Slough.

slough

Ah, the slough of despond. In UK English, it rhymes with cow. In US English it rhymes with pew. (I used to live near Midshipman Slough, near Tubbs Island, in Sonoma county.) I spent a lovely week in Slough, England (near Maidenhead, Windsor, and Eton). The so-called poet, John Betjeman, wrote a famous poem starting:

I was thinking of the verb form.

In California I heard it pronounced as 'slew'.

I don't think it was Slough, but there's an area in England where there are three towns, all with the same name, at least by spelling, ending in ough and they're all pronounced different. I think the pronunciations are 'oo' (or maybe 'oh'), 'uff', and 'ow'.

-ght

The quick answer is that words in English ending in -ght fall into three groups: (1) verbal forms where the -t is a suffix in Old English denoting tense/aspect, (2) words that go back to PIE roots ending in -k(w)-t, and (3) miscellaneous.

In group 1, you have words like bought (< buy), brought (from bring), sight (from see), wrought (from work), etc. In group 2, words like night (cf. Latin nox, noctis, Greek nuks, nuktos), wight (and aught, nought, too, < PIE *wekti- 'being', cf. Russian veshch 'thing'). In group three, some words with unknown etymologies, e.g., blight. (There is some overlap between groups 1 and 2 because, it may be the same process but simply a matter of when it happened in the Old English, Proto-Germanic, or PIE period.) The main reason for supposing a suffix PIE -t- is that PIE roots are usually of the form CVC (simplified, with V covering a lot of other kinds of sounds than vowels). You can see this in the examples light and night above. Cf. Latin lux, lucis, 'light' which does not have the -t-, but German Licht, English light, which do.

Wow! Thanks, gives me lots of things to look up and research.
I really appreciate your taking the time to give me some
sort of 'quality" answer. Grateful.