<10% of disk space available is a big hit.... OSX dynamically sets the swap file and if the disk space check which occurs prior to (re)allocation indicates it is below a certain threshold (unsure of exact %) the OS limits itself.
What applications do you have starting at logon? Is spotlight indexing all of the time (magnifying glass icon has a dot in the center of it)?
Also if you have really large files, like movies, the harddrive may be fragmented. Don't flame me, I know HFS+ supposedly deals with fragmentation on it's own, however with large files it still happens. The way to deal with it is to copy the files off to an external drive, reboot, run the daily/weekly/monthly maintenance scripts, reboot and copy them back.
Try moving whatever it is you have which is so large - your iTunes library perhaps - to an external HD, reboot and see what happens. I would expect a big increase. If not then there is something else going on; reinstall the OS.
A step of last resort you can try before reinstall is to run all of the cleaning steps in Onyx (wipe all of the caches). Again reboot and see what happens.
Isn't the Bill Gates quote from the 80's "nobody will ever need more than 640k of memory".... The S Jobs corollary today is "who needs more than 80gb of harddisk space."