Provided you use a unique home directory name for each distro, you can use the same /home/ partition for all the distro's. This will allow you to have a larger home partition. Also, if you configure the distro's to delete all of the files in /tmp when shutting down, that is another partition that can be shared. Having around 10GB for /tmp can be handy if you want to burn dvd's. Of course as already mentioned you can share the swap partition. The other distro's will notice it and may even use it during the install process.
Look at your current /etc/passwd file. The home directory of the user is given there. So you can use the same user name but have different home directories such as /home/username-fc9, /home/username-suse, etc. Keep in mind that access is determined by the UID number of the user and not the user name. So in Fedora Core, the first user will be 500 while in SuSE the first user will be 1000. A user with a UID of 500 in other distro's will be able to access the FC's users directory with a uid of 500. You can edit the regular UID & GID's range of numbers by editing /etc/login.defs.
# Min/max values for automatic uid selection in useradd
# SYSTEM_UID_MIN to SYSTEM_UID_MAX inclusive is the range for
# UIDs for dynamically allocated administrative and system accounts.
# UID_MIN to UID_MAX inclusive is the range of UIDs of dynamically
# allocated user accounts.
# Min/max values for automatic gid selection in groupadd
# SYSTEM_GID_MIN to SYSTEM_GID_MAX inclusive is the range for
# GIDs for dynamically allocated administrative and system groups.
# GID_MIN to GID_MAX inclusive is the range of GIDs of dynamically
# allocated groups.
This will effect new users created.
You might consider creating symbolic links for your Documents & downloads directory. This will allow you to access the same Documents easily which ever distro you try.
One thing to consider is that some distro's use LVM and others don't by default. If you are going to use LVM, you may want to start with a distro like Fedora Core that uses it by default. This will create a /boot partition. From then on you can use the FC's grub boot loader to chainload the other distro's.
You will want to have one main boot loader on the disk's MBR. From there you can either cut & paste from the other distro's menu.lst file to boot using a single /boot partition, or have chainloading entries to boot the other distro's using there Partition saved MBR's. You don't want to get in a situation where you have dueling distro's updating the same MBR.