There are quite a few flavours of UNIX free for download and use .. notable are FreeBSD [www.freebsd.org] it is available for download from FTP mirrors. The list is also at:
http://www.freebsd.org/handbook/mirrors-ftp.html
And then there is OpenBSD which is very secure and available from
http://www.openbsd.org/ftp.html for list of sites. Just examples of 2 UNIX distribs.
Regarding partition managers? Paying for software is the Windows way of life
.. the NIX come with very safe and easy to use partition and boot managers [dont know which you mean]. The NIX use fdisk which is perfect as opposed to PartitionMagic and M$-based fdisk, and FreeBSD uses Booteasy as the boot manager.
No matter which NIX u go for you are covered, just follow the instructions and u'll be fine.
Cheers