Im developing web-application with Merb and im looking for some safe and stable image processing library. I used to work with Imagick in php, then moved to ruby and start us
This is not due to ImageMagick; it's due to Ruby itself, and it's a well known problem. My suggestion is to split your program into two parts: a long-running part that allocates little memory and just deals with the control of the system, and a separate program that actually does the processing work. The long-running control process should do just enough to find some work for a child process that it spawns, and the child should do all of the processing for that particular work item.
Another option would be to leave the two combined, but after a work unit is complete, use exec to replace your process with a freshly started version of the same program, which would search for another work item, process it, and exec itself again.
This is assuming that the work items are fairly large, which they almost certainly are if you're using ImageMagick. If they're not, you'll find that the overhead of spawning a new process and having the Ruby interpreter re-parse your entire program starts to get a little too large. You can deal with this by having your program do more work units (say, ten or a hundred) before re-executing itself.