Boktai: The Sun is in Your Hand was released for the Game Boy Advance by Konami in 2003. The brainchild of Hideo Kojima, father of Metal Gear, Boktai is similarly a stealth-action game where the player is better off running behind or tricking the enemy than trying to win everything with a firefight. The key difference is that instead of a secret agent, the main character of Boktai, Django, is the son of a vampire hunter who uses the power of the sun to banish evil from a scorched land.
Boktai 2: Solar Boy Django is its sequel.
The most noticeable thing about Boktai is that the cartridge itself has a transparent case and a large, bulbous protrusion from the top (or bottom, if it's plugged into an SP model). This is not a cancerous tumor, but the housing for Boktai's solar sensor. The sensor registers sunlight, and can tell the difference between natural light and most artificial light. Boktai is designed to be played outside.
The solar sensor charges Django's main weapon, the Gun Del Sol, which uses solar energy to, er, purify zombies and ghosts and the like. Treasure chests scattered around the game world allow the pickup of gun parts to change the alignment (like ice power and fire power), beam style (slow but powerful shots, rapid-fire, a solid beam, et cetera), and other features of the gun.
Unlike Metal Gear, Boktai is conceptually split up into level-like dungeons (though the dungeons are themselves connected by an MG-like overworld map). The dungeons are filled with both enemy encounters and puzzles; however, the encounters themselves are puzzle-like in the endeavor to kill as much as possible without dying. Each dungeon has a boss, an immortal servant of evil who must be purified to avenge the death of Django's father and save the world and everything. Boktai uses an interesting mechanic for boss fights - after a traditional battle with the boss monster, where he or she or it unleashes special moves and generally tries to knock the crap out of Django, the immortal is sealed into a coffin, which must then be dragged via a chain over the shoulder back out of the dungeon, usually using special escape routes set up by puzzles.
Once outside, the coffin is placed on a circle of purification-schmurification (a "piledriver") summoned by Otenko, a sun sprite accompanying Django for this express purpose, in the front lawn of each dungeon. Using the power of the sun, Django activates some radar dish structures that concentrate solar power, which proceed to pummel the coffin into submission. This is actually a second fight - the immortal's spirit will attempt to fight back through the coffin, trying to immobilize Django or deactivate the dishes, as if it manages to deactivate them all the coffin will try to worm its way out of certain doom. Succeed in keeping the dishes active for a while and the immortal is no more and you win.
Obviously solar power is a large part of the game, as it fuels Django's ability to kill things, albeit the complication that most of those things are already dead. It also serves to light up windows in indoor areas, which can be used to solve puzzles and set enemy traps. However, another large part of the gameplay is the time of day, and in fact the time of year. When starting the game the player is prompted for the time, date, and nearest physical location. Intelligent algorithms within the game use this information to create realistic day-and-night times, usually accurate within minutes to real-life dawn and dusk. Playing at night-time is a different experience than during the day, as zombies will be more active when it's dark out. Therefore the advantage of the sun is not only in its power to make enemies defeatable (though most rooms in the game can be solved without using a bit of sun power, immortal encounters require it to fuel the piledriver), but also in that the game is easier in daylight hours. Even the phases of the moon play a role in some in-game events.
This of course leads to the shortcoming that, in order to experience the full game, you need to get outside. This is discouraging to most video game players, especially those who recognize the name Hideo Kojima. The game also suffered from an extremely weird advertising campaign. However, its clever innovation makes it a fun and unique GBA experience, provided you have the sunlight to back it up.