I was browsing through the PSM store just now, looking for any games I'd picked up but long since deleted to regrab before it closes in a couple of months (15 July). While checking if there was anything else worth picking up, I noticed quite a few free games there, and what's not to like about a free game.
The store opened two-and-a-half years ago, as Sony tried to ride the wave of Android games and bring some smaller games to the Vita. In that respect, it worked as there's a lot of content there, some great, some good and some amateur - but that was the point. However, Sony soon started to ignore it, dropped Android support and once it had grabbed a updates of the better games (Aqua Kitty, Rock Boshers, etc) for the main Vita store, all but gave up on it.
So, before this useful resource vanishes into Sony's desktop trash can, give the PSM store a visit, redownload your old titles, grab the free ones you can and pick up some of the goodies your missed. Many are less than a £1, so what's not to like. There's many more than my little lists, go check it out.
TOP 10 PSM TITLES
Super Crate Box
Life of Pixel
Aqua Kitty (Get the trophy rich DX version on the main PSN store)
Rock Boshers (Get the trophy rich DX version on the main PSN store)
Hermit Crab in Space
FREE PSM TITLES
UPDATE: Life Born (Takuya Nagasuku) Released 20th May, a tilt game all about evolution
Shuttle Quest 2000 (trial)
Massoumano Saving Latifas
Super Duck (a bit like Lemmings, but with ducks)
Everbody's Arcade (Trial)
Cytus Lambda (rhythm action)
Ninja Insect (augmented reality
Galactic Junk (video)