Today's review is for Abuse, a gpl'ed sidescroller (surely you know that means the source code is available), with public domain content, which is strange and unusual, and also means the entire game can be downloaded free of charge, here.
The linked page hosts a Windows port of the game, and that is what this review is relevant to, but ports exist for many other operating systems, and if you're running one, you probably know how to use google.
Abuse steals pages from the respective books of Metroid and Contra, and adds mouselook, making it the only 2d platformer I've played that supports (hell, requires) a mouse and keyboard control scheme. This sounds awkward, but mouselook works unexpectedly well in this style of game, and makes Samus and the Contra boys seem like poor marksmen indeed.
While it plays like a hybrid of the two console classics mentioned above, save games are implemented, as are quicksaves, which gives you the option of playing through console style, saving only when the save points come up, or creeping through as though you were playing Quake, saving every few feet. You'll probably end up doing the latter; the difficulty spikes relatively early.
Oh yeah, there's a story: You're the lone survivor (aren't you always?) in a prison taken over by a failed experiment and filled with mutants with laser beams attached to their heads. This is a paraphrasing, but you get the idea. Cue dark, spooky corridors and explosives.
Which sound like explosives should: bang. Sound effects are on point and the music is mostly atmospheric. I believe that with a few exceptions (such as the Super Mario Brothers overworld music), the less you notice the sound in a game, the better it does the job of involving you, and I hardly noticed the sound in Abuse.
Abuse offers network play over TCP/IP, serial cable, modem, or IPX. (Special note to coders: Make a client/server mod for Abuse so we can have 32 player 2d sidescrolling deathmatch! Please? It worked well for zDoom...) Now, all of the above is impressive for a free game, but it also includes a level editor. So you can make your own killing grounds, or even single player levels, if you're lame.
Anyway, Abuse is a little under 3 megs, and packs a whole lot of game. I recommend it. If you're feeling frisky, try the newer version, which adds higher resolution graphics, also linked to at the above page. By the way, I've seen the author of this port post to slashdot a few times, so big ups to you for the fine effort, Jeremy Scott.
update: Abuse actually takes up a little over 9 megs on the old hard disk. The download is less than three megs, though