I love a remake of a classic game, particularly when they are free!
Post your favourite remakes and Mods here:
Remakes
UFO Alien Invasion: Link Not tried this yet but looks pretty promising. FAQ says it is based on a modified Quake II engine.
Diaspora: Link A complete overhaul of Freespace 2 with a Battlestar Galactica theme. You don't need the original Freespace 2 to play it. It's incredibly good.
Dizzy Link It's Dizzy with a bit of a facelift. You can't go wrong.
Settlers I & II: Link Another one I've not tried but supposedly not half bad.
Kings Quest I/II/II Link All seem pretty snazzy. I wonder how long they'll be available to download though.
Space Quest II Link As above, I wonder how long the download will last.
Free Orion Link In the "tradition" of Master of Orion.
Panzer General Link Excellent strategy game.
Black Mesa Link It's Half Life in a new engine. It's totally free and it's very very good.
Open TTD Link TTD is like crack cocaine for those who have played it. This is just as good.
Freeware Releases
The Derek Smart Collection (Battlecruiser et al) Link to all of them Derek Smart is a legend (in his own head. Can't find a link to the original Battlecruiser Flamewar but the comments on this article will give you an impression of him. It's cool that he got involved and even joined a game with some people, but the product just wasn't as good as he seemed to believe it was). Play Battlecruiser and Universal Combat for free (and at your own peril)
Beneath a Steel Sky Link I am guessing most people have already got this, along with GoG's other freebies. You should certainly try it if you haven't.
Starseige: Tribes Link (Cnet) Decent FPS. Fun with friends.
Hidden and Dangerous Deluxe Link (Shacknews) Tough, a bit buggy but worth a look.
C&C Tiberian Sun Link (Shacknews) Enjoyable RTS.
C&C Red Alert Link (Shacknews) More RTS
C&C Link (Shacknews) The original, a bit dated now.
Elder Scrolls Arena Link (Cnet) Not the best game in the series.
The above links relate to genuine freeware releases as far as I know. If the mods don't like them here I'll chop them out ASAP.
I am sure that there are more hidden gems out there somewhere...