Title says it all really. What's the last old game you installed (and hopefully played) that isn't yet on GOG.com and roughly when was it? The last one I installed was yesterday and it was Sid Meier's Sim Golf.

I think the last one I did was Terminator Skynet, to see how well it ran on Dosbox, about February.
If it's the last non-gog game I installed and actually played beyond testing performance I think it'd be Albion which I tried to get into around November last year...I failed once again.

Commander Keen in "Aliens Ate My Baby Sitter!"

Was playing around with DOSBox, testing some different settings. Played a little bit, and it's still awesome :) Commander Keen 4-6 were the first PC games I got really hooked on. I was 10 at the time :P

Edit:
And I'm still hoping to see the entire series (including the above-mentioned #6, which has become the victim of the scattered-licenses-syndrome and cannot be legally obtained anywhere as a result) on GOG.com some day.

Star Trek: Elite Force, because I had a craving for some Star Trek gaming and I didn't want to pay for Star Trek Online.

Recently reinstalled Starcraft and Starcraft: Brood War. I never completed the expansion back in the day so I felt a need to pull out the CDs. Good times!

Hmm I believe that would be either "Syndicate Plus" or "Quest for Glory IV: Shadows of Darkness".

1-5 are availble on Steam. I'd quite like to see 6 as well. And never figured out where Keen Dreams fit in.

Strife

Dungeon Keeper 2 for me. Well technically I have Dungeon Keeper 1 with Deeper Dungeons installed as well, which GOG doesn't have yet.

Overlord

It is pretty much the only game I have installed right now. :p

Dungeon Keeper 2

Unfortunately can't pass Level 4, as it instantly crashes all the time at the same event in that level :(

Yeah, I know they're on Steam, but I'm a patient guy and I'd rather get them here if/when they arrive.

Keen Dreams was developed as part of an obligation to Softdisk, the company where John Carmack, John Romero and a couple of other guys were working in the daytime while they developed the first trilogy of Keen games in their free time. After releasing those first three episodes with the help of Apogee Software and getting their first royalty check, they resigned from Softdisk, started id Software and moved on to develop Keen 4-6 , Wolfenstein 3D, DOOM, etc. etc.. Keen Dreams (aka Keen 3.5 or The Lost Episode) was produced for and released by Softdisk as part of contractual obligations and falls, technologically as well as in terms of when it was produced, in-between episodes 3 and 4.

Zork Grand Inquisitor, which I still hope will apear in the GOG-Catalogue - since this game has some issues on modern machines.

Dark Sun.... Got an itch for some old school D&D.

Last? Star Trek 25th Anni and Judgment Rites.