tfishell: I noticed that one of the top reviews mentions SC4 modding, and figured I'd ask if somebody wanted to write up a detailed explanation on how to get the mods working, recommend some, provide links, etc. (Maybe it's not necessary, I personally don't know.)
I was thinking of writing something, but I'm not really all that knowledgeable and was mostly going to quote/paraphrase this, anyway:
http://community.simtropolis.com/topic/36525-top-ten-modd-suggestions/page-3#entry1381771 That's mostly gameplay fixes and convenience tweaks. Note that I don't have the GOG version, so can't really say if any of these are included (unlikely, knowing GOG practices). To that list, you might wish to add some of these:
http://community.simtropolis.com/topic/49337-mandatory-fixes/#entry1247099 As to mods proper, first and foremost I'd say you want the
Network Addon Mod (NAM). After that... hard to say, it's really an open field. You might find the following thread useful, but I'm fairly certain I've seen better ones out there:
http://community.simtropolis.com/topic/36525-top-ten-modd-suggestions/
Hope that's at least somewhat helpful. SimCity moddery is, predictably, very serious business ;)