are they any free modem speed boosters (that realy work !)
out there ?

If you're reffering to the speed increase claims that you see on TV or in other ads (usually, saying that they are 5 times faster than dialup), they actually don't affect your modem at all. For example, the NetZero Highspeed cannot make data travel through your dialup connection any faster than somewhere around 56Kb/sec. What it can do is make sure you connect to one of their fastest computers with the least load. It also uses caching and compression to speed up your connection. Usually, the webpage is sent to the Netzero computers, where everything is compressed to make it smaller (this can sometimes result in poor quality pictures, depending on your settings). Because the webpage is smaller after the compression, it seems like your connection is "5x faster". Typically only web browsing is affected, and downloads are downloaded at the same speed as before.

In answer to your question though, as far as I know, there aren't any accelerator programs for free. Check with your ISP to make sure that they don't offer one. AOL offers all their users an accelerator inside their browser, ATT offers an accelerator either for free or for a small fee, depending on your account. Netzero offers theirs for an extra $5 a month.