EASER  I honestly wonder if this works. I ran VMfusion, which ate up pretty much all of my remaining memory. After I quit, I ran iFreeMem. Although the graph visually says I "recovered" a lot of memory, my computer still ran about as sluggish as it did right after I quite VMfusion. It most certainly did not run as fast as it would if I restarted.   
(Version 3.0)

GERWINPHILIPPO  "Defragment memory" ? That must surely be a joke?! That's just proposterous! Fragmentation is a mechanical phenomenon that cannot, by definition, occur in memory. This is definitely snake oil. You must be a fool to actually pay for this rubbish.  
(Version 3.0)

FOULGER  Sure there are plenty of Applications/Utilities that do the same thing and one can enter a unix command & also optimise RAM but ifreemem is in the menubar, memory use can be seen and clicking on optimise takes just a small amount of time. For me that saves money each time I use it. The author has offered it for sale, if you like it then pay for it, if you don't then use something else. Its just so convenient and can be used with very little waste of time.

As a codicil, I would, however exhort my oft used advice "Halve the price & triple the profit"  
(Version 2.5.1)

NARBO  sudo /usr/sbin/periodic weekly

produces the same results for FREE.  
(Version 2.5.1)

HOLYPOLY  Leopard Cache Cleaner does that (and MUCH more) for $8.99 .   
(Version 2.5.1)

XPLICIT  Thanks Dana for telling that it works as advertised, because I get slow downs on a MBP with 2GB RAM. That is without even starting the "killer" apps, like PS and such. And I don't agree to 2GB not being enough memory.  
(Version 2.5)

DANA SUTTON  Well, for better or worse it works as advertised. Use it, you get significantly more free memory. But nearly twenty bucks for a gizmo like this is i. m. h. o. unacceptably steep. I'd spring for about five, tops. If you have so little memory that you really need it, you probably would be better advised to start saving your pennies and buy more RAM.  
(Version 2.5)

MAC-KIWI  If you clean the cache-folders by an apple-script, you have the same effect. Applescript is for free and integrated in OSX.  
(Version 2.0)

PHYSICSGUY  You shouldn't have much free memory. What you want to make sure of is that your free + inactive memory isn't low.

The reason people see slowdowns when they run this program is that you're effectively deleting a bunch of cached data (like Safari tabs you aren't currently viewing) from memory, so you have to reload it from disk.

Here's an Apple document that goes over the types of memory you see in Activity Monitor and what they mean: http://docs.info.apple.com/article.html?artnum=107918  
(Version 2.0)

FOULGER  I've been using the trial for two days and it appears to be making things more zippy. I'll use it till the trial expires and if I conclude that it speeds things up overall I will buy it.  
(Version 2.0)