Cleanmem does not actually do anything, it merely asks Windows to do it the proper way, thus there is no lag like with other mem cleaners.
Memreduct is probably the best free one out there, but it does not have a digital signature, so not really suitable for MT members.
No, the RAM is used like any other time, like when you close the app and memory gets freed.
Not anymore, I install it on systems with little RAM, like 2-4GB, but I used for years before (it helped), even a similar app called Cacheman since XP.