Hudson Soft's Rengoku series were two of the worst rated Playstation Portable games back in the day. I had a fun time with both of them: Great technical hack n slash combat with tons of customization. The second one is the way to go, since it features a new tower and a remastered version of the tower from the first game. Also, level design is much better, the first one was fully orthogonal design.
My biggest complain is the difficulty, both games are way too easy, unlike reviews said back in the day. Once you get a good gun to do post-mortem finishers (the first machine gun, for example) it becomes a walk in the park.
Team Tachyon's Quantum Theory. Maybe the first obvious Gears of War clone. It was certainly crappy when it comes to graphics and overall quality, but the gameplay was ok and fun. It was developed by Team Tachyon, an internal team of Tecmo Koei, developers of Rygar: The Battle of Argus for Wii and the great Undead Knights for Playstation Portable.
Creoteam's Collapse. And yet another Gears of War clone, so pretty much the same as Quantum Theory, crappy technical quality, fun game. This is curious because of the ukrainian-based developer, Creoteam, which sounds suspiciously similar to Croteam, the croatian-based developers of Serious Sam series. And another fun fact: Here in Spain it was localized by the famous studio FX Interactive. It was called After the War, in order to be sold as a fake-sequel to the classic Dinamic Software eighties shooter (After the War). Of course it had nothing to do with the original game.
It had a sequel, The Rage. I havent played it but looks pretty much the same as the first game.
AlterLab's Skalolazka. Russian game based on a russian film, localized in Germany and France as Vera Jones (for Indiana reasons). It is probably the crappies Tomb Raider clone I have ever played, but still, it is fun and has a clever, yet barely working, climbing system, that probably led to the Bennett Foddy's QWOP formula. There's an English translation on Github.
Vitei's Rock and Roll Climber. WiiWare has tons of small, weird and cool games. This one.. maybe not. But I love climbing games and Rock and Roll Climber has Wiimotion + Balance Board support. The climbing system balancing the body and using the Wiimote/Nunchuck is pretty cool, unfortunately the game is very hard and very glitchy. Rock and Roll + climbing because why not.