If i join the playstation plus, will I be able to download the free games ?

A lot of the Playstation Plus sales are temporary. But as long as they're still available and the Playstation Plus sale/price still applies to them, yes.

Nope only what the store offers at the moment I'm afraid.

no, u can only download whats currently free while ur Plus is active, and u can only play them whele ur Plus is active when it runs out u cant play them again until u renew ur Plus, games u pay for thats on discount u can still play even when ur Plus runs out.

There are many big and small ps3 games that are free for ps plus users like infamous 2 and mass effect 3 and you get many other things like themes for free too and many things are discounted for ps plus users but there is a thing you can keep the free games but you'll only be able to play it till you are a ps plus user this dosen't effect other things

Probably so but most of the games are free for ps plus users and some are half price.