Remember Warning Forever, the single screen boss rush shooter with the boss that 'evolves' over time? Picture that, but with shinier graphics, and instead of evolving bosses, you get a rush of wackily pre-made bosses to fight. That's Fraxy. It's a boss rush sandbox, and a really well made one.
You fly a neat looking ship which moves in all directions using the cursor keys, and aim it using the mouse. A very nice effect is that the ship achieves its motion with thrusters, but it uses the correct ones at all times, so if you move and rotate you see the various thrusters turning off or switching on as appropriate for its angle.
The ship also has a variety of weapons, most of which are overpowered (this has been balanced in the new beta). It also has an odd power system; you have an amount of power which is depleted by various actions such as firing or boosting. This power also protects your ship, so if you get hit by weapons, you lose power. If you get hit while at zero power, you die, so your weapon power doubles as your health.
This actually works. It means that you can't just fly in blasting away without thinking about it - if you fire off a high-power blaster weapon, you're leaving yourself less well protected. Luckily the power regenerates, so all you have to do is fly away and rest.
But the game's main feature is bosses. It comes with about twenty pre-made bosses, and a boss editor to add your own. The editor is amazingly good. The bosses are made in Warning Forever style tree-form - you begin with a core, then add bits to this, and add bits to those bits. It's a lot more versatile than it sounds, and it also makes it easy to have weak points and chain-explosions on a boss (destroy a piece close to the core, and all pieces attached to it fall as well). The editor even has a simple scripting engine if you want the boss to respond to actions, but most of the pre-made bosses don't even need it.
Disadvantages? It gets a little samey after a while. The bosses you can make are great, but there isn't much to distinguish them once you've fought them a few hundred times. Maybe persistent debris would make it more interesting; blasting a ship in half, for example, only to have both halves still be attacking you. But for a quick blast, it's great.
It's mostly Japanese, but the author has done a passable job translating for English speakers.
Fraxy download page
Stable version of Fraxy from 4th February 2009
Beta version of Fraxy from - today! (which didn't work on my machine)
The beta version is noticeably better if you can get a version which works (it has a more zoomed out view so you can see more of the bosses you're fighting), and I haven't found any bugs in it yet.