The "Gold" CDs are usually CDs made by one of a few companies including DCC (now defunct), Audio Fidelity, and Mobile Fidelity Sound Labs. Steve Hoffman mastered for DCC and now masters for Audio Fidelity.
SHM is a different audio CD format, along the lines of XRCD, meaning backwards-compatible with Redbook CD hardware but somehow able to offer "audiophile sound quality". SHM CDs are usually produced in Japan.
"Remasters" of old music are continually being put out all the time - usually of music from the 50s-80s.
Here's the real deal on the gold CD / remaster business:
Audio Fidelity and MFSL are the two main companies putting out "Gold" CDs today, which really means nothing - it just means the discs have a faux-gold coating on the data side. As you've noticed, there are tons of CDs they've made to date - and the discontinued ones often go up on eBay for as much as hundreds of dollars. Supposedly some of the discontinued ones actually sound pretty good, like Pink Floyd's Dark Side of the Moon, for example. I have a lot of OOP (Out Of Production) DCC and MFSL Gold CDs myself, from bands like Pink Floyd, Steely Dan, The Who, R.E.M., The Cars, Deep Purple, etc. A lot of them I haven't listened to though so I can't really speak to sound quality.
In all cases, supposedly the gold CDs are always "remastered from the original tapes" which means optimal sound quality from the original analog master, but I've personally found cases in which users have reported this to NOT be the case. See Amazon reviews of every gold CD you're thinking about buying - some of them have really informative customer reviews about this. Also, I've seen many reported cases of Steve Hoffman not accurately retaining the source music, sometimes editing out portions of songs, or rearranging songs, etc - again, you have to look out for these on Amazon. This got so reported about that I decided to stop buying Steve Hoffman-mastered CDs about a year ago.
Sometimes other 3rd parties will also remaster old music onto regular silver CDs, you can usually spot them on Amazon if they have a year on them, like CTI's 2010 remaster of Freddie Hubbard's Red Clay. The 3rd-party remasters can be hit or miss, it definitely pays to read the Amazon reviews. I've skipped over a lot thanks to Amazon reviews.
On the OOP Gold CDs that I've bought on eBay, I never spent more than $80 for one - but that's just me, others will inevitably have lower or higher budgets than that. I used auction sniping a lot. 
I've read reports that supposedly SHM is high quality but I can't speak for that myself yet.
XRCDs are popular too, and I have a lot of them, most of them either OOP or from the AudioWave Blue Note series (jazz music). In my experience, the XRCDs usually sound really good - not that I've done any comparisons to regular CDs, but I had nothing to complain about with the XRCDs.
Determining mastering quality for yourself on any CD is pretty simple. Rip the CD on your PC to uncompressed WAV (not sure about the procedure for Macs) using ExactAudioCopy and load the WAV into a program like Audacity to view the waveform and spectrogram. More info on this in general can be read here: http://en.wikipedia.org/wiki/Loudness_war
As for buying the various CD formats, this is where I buy mine from:
- Analogue Productions SACDs: Amazon.com
- Audio Fidelity Gold CDs: ElusiveDisc.com, Amazon.com, & Spun.com
- MFSL Gold CDs: ElusiveDisc.com, eBay.com (for OOP CDs), & Spun.com
- XRCDs: ElusiveDisc.com (have used MusicDirect.com and AcousticSounds.com in the past)
In the end, just because something is a remaster doesn't mean it'll sound good. Definitely pays to research a specific CD before you buy it. I've had pretty good success with 3rd-party remasters though, mastered either this year or last year, so far. And I haven't gotten any clunkers from MFSL either yet, so I'll continue to get those. But I've since sworn off AF Gold CDs due to all the reports of substandard work by Steve Hoffman.
