Two very interesting sites looking at the development of eroge in a historical context come to my attention. Firstly, and most strikingly, long time visual novel producer Alicesoft has actually officially authorised the distribution of many of their older titles (1990s) as abandonware; you can find them for direct download or torrent download, collected here. Secondly, there is a compendium of older eroge collected nicely with some CG (see below).
I think I will not be alone in applauding Alice for this: it is nice to see abandonware games get recognition from publishers.
The other site is rather more accessible. At “Let’s ヲタck ‘n’ Roll ♪” (ota-), you can find a most interesting compendium of top rated (by Megastore magazine, no less) eroge by a variety of developers, each with some representative CGs.
Period covered is the mid-90’s, with there being some recognisable titles present.
Finally, you can see how far the art of Alicesoft has come – quite the progression?