In my honest (Really, really honest. Honest.) opinion, GameSpot is THE best game review site around.
With over 3000 games, Home of the Underdogs is the most comprehensive abandonware site around.
Game Hippo hosts over 1000 freeware games, and has a short review of each.
Free Games Net is a site similar to mine. It features shareware and freeware games, with a fairly comprehensive review for each of them.
One of the inspirations for my first website was Electric Games. Unfortunately, it hasn't been updated for a long time and many of the links are broken, but it's still a good source of shareware and freeware games.