Alien Arena is a frantic Quake-like FPS game made from the open sourced Quake 2 source code.  In short, the action never lets up, the graphics compliment the game's theme, the sound design well illustrates the game, and the game is polished to the point of being blindingly shiny.

Graphics:  Back in the day, the game sported a spiffy 50's retro look, but with new releases, this has changed.  Now, the game takes on a more industrial like setting, with an entirely new level of map detail to accompany it.  Some of the maps just look stunning.  Using SVN, you can get the 'between releases' game, which has new player models in the works (one highly detailed martian enforcer model is already done), new maps that are even better looking than the old ones, and even new weapon models (the new rocket launcher is done).  There are also modern graphical effects, like light bloom, Dot3 bumpmapping, realistic water reflections, and even real time lighting.  However, some of these features come at the expense of your framerates, and some of them are fairly undeveloped (the bumpmapping isn't complete yet).  Also, some of the textures look fairly grainy.  But overall, it's good.  9/10

Sound:  The sound design fits the wacky style the game had in releases past, and even with the radically different theme, they still fit the game.  You can tell what things are happening just by the sounds (always good), and much like the regular Quake games, they can still be used to plan accordingly in 1v1 matches.  You also get the feeling that you're in the universe with the ambient music, that ever so subtly makes the game feel better.  This doesn't stop some of the sounds from being a bit out of place, however, such as the disrupter/beamgun fire sounds.  Some of the weapon sounds are also just small blips, like with the blaster.  But in the end, it doesn't really matter all that much, because you get absorbed in anyway.  8/10

Gameplay:  Gameplay is where Alien Arena shines when given the chance.  Unlike many new FPS games, Alien Arena focuses on fast paced deathmatch instead of slower, tactical action.  So, even though some people may feel alienated (no pun intended), when given a chance, the action is much more intense than in similar games.  There is also a lot of polish in this area.  The weapons all feel like you're actually firing them, unlike in other games, and the newest release, 6.10, featured a Quake 3 style ammo/weapon pickup system, that made the game flow better.  And all this polish gets shown off in multiple game modes.  There's your standard deathmatch, team deathmatch, and capture the flag, but in addition, there are some lesser modes.  Deathball is almost deathmatch, except there's a ball that you can take and put through the goal for extra points.  All out Assault is deathmatch, but with vehicles, some of which haven't been touched and improved upon since their initial release, so this may be a little odd when you first start out, but the vehicles add a new focus on the Z axis, and the gameplay is more diverse because of it.  Team Core Assault is a team mode (well duh), where the goal is to disable the enemy team's three power nodes and then destroy their power core, while preventing the enemy team from doing the same.  Lastly, cattle prod is a silly mode where you and your team guide cows to the capture point.  There are also multiple mutators, such as instagib, rocket arena, and joust.  But all of these things would be wasted if the gameplay couldn't properly integrate with them.  Luckily, this isn't the case.  Each mode feels as natural as the last, and all are fun to play.  On a more advanced note, the game keeps up the Quake tradition with trickjumping.  The normal things such as strafejumping/bunnyhopping, rocket jumping, plasma climbing, etc are all included for the players that can do them, but even niftier is blaster jumping.  This gives the blaster a use other than weakly firing at opponents and hoping to whittle them down.  Lastly, the weapon balance in the game is good, but not perfect.  So, there is some incentive to nab the vaporizer/beamgun and whore it all over everyone, but skilled players can get around this and bring the match back to the norm.  It's all intense, teeth grinding action in the end, something that most freeware games fail to pull off properly.  9/10

Various:  Alien Arena sports a helpful community surrounding the core to make the game even better.  Weekly tournaments are organized, and clan matches are a norm in the community.  Of course, just like anywhere else, there is some drama, but nothing too terrible to drive you away.  Also, to address what the other reviewer said, the two executables are entirely different.  One launches directly into the game, and the other is the multiplayer server browser.  Both are good enough for getting into a game, but the server browser, named Galaxy, also has ways to check your player stats and even chat in the IRC channel.  This is where the community hangs out, and if you have problems with the game, such as the ones the other reviewer mentioned, like having trouble getting into a game, just ask in there and someone is sure to help you.  Even the guy who constantly develops the game hangs out in there, so you can tell him (Irritant) any feedback you may have.  He's glad to hear it.

Overall: 9/10 (not an average)
Not absolutely perfect, but better than most of it's competition.  GrueTamer out.