I feel like I've said this before, but if you have Diablo II: Lord of Destruction, play Median XL and/or Ultimative; there's also Eastern Sun, and a couple total conversion mods that may also be worthwhile.
One thing to note about Median is that there's level challenges that only grant rewards when you're underneath a specific level, which isn't very enjoyable in my opinion. Ultimative has 1 more of that, iirc. And you may find yourself reading documentation of the mod a lot at the start.
Also second the suggestion of Deus Ex. First thing that comes to mind for me is the story and exploration. (I think the Nameless Mod is worth mentioning, but I haven't tried it myself)
Deus Ex is good. Played it on OnLive. I got it free along with Deus Ex: Human Revolution Augmented Edition. I think it's better to buy it on Steam. My main issue with OnLive is the fact that you must be online at all times even for single player games. So, know any interesting mods for Deus Ex?
Onlive works that way because you're pretty much remote controlling a more powerful computer and streaming the audio and video via the internet over to your computer. It's good because, any old PC/Mac/Cellphone/Tablet can access the game and let you play as long as you have a connection.
Deus Ex is also on GoG, but I have the Steam version, which still can be run independently from Steam. According to GoG's store page there's some extra goodies (e.g soundtrack) that come with Deus Ex. Deus Ex on Steam also comes with the totally sweet soundtrack but they're in .umx.
I used the NewVision mod which is for graphics (resolution, aspect ratio, texture quality, frame rate, FOV). The listing of mods on ModDb looks interesting, but I haven't tried them out yet.