I just downloaded this game: Seiklus and despite the simple graphics, it’s a fantastic game; there are no clear objectives, you don’t get killed, there’s no time limit, but yet it is compelling and addictive.
NB: it’s a manual install; unzip to a convenient location and run the .exe file - on winXP, I had to right-click and tweak some of the compatibility properties to make it run at more than a crawl.
On my website I have a page full of game links, most of which are freeware or shareware. Enigma and Head Over Heels are a couple of really good ones, IMHO.