I suspect the free 30 day trials are only for the current major release versions (which will not run under Mac OS X 10.5.8) although you might want to contact VMware Sales directly.
If you want to give Virtualization a try under Mac OS X 10.5.8 and you can't get a free 30 day trial of a version of VMware Fusion that's still supported and runs under it then I'd suggest trying VirtualBox and is free! 
The last version of VirtualBox shown to support Mac OS X 10.5.8 was 4.1.22 and I've included the relevant links below for your convenience as this is not a VMware Product nor a support forum for VirtualBox however I just wanted you to have an option if need be. 
Download VirtualBox 4.1.23 (98MB)
MD5 Sum: 708512613d2ffa4908ad0a089ad4d475 VirtualBox-4.1.23-80870-OSX.dmg
(Note: The Mac OS X 10.8.2 release includes some incompatible changes which require adaptions in VirtualBox. Therefore the original 4.1.22 build was replaced by a 4.1.23 build. However this should still work with Mac OS X 10.5.8)
Download Extension Pack for 4.1.22 All Platforms (10MB)
MD5 Sum: dafb1a66c0f88b7ffecc2b325223b669 Oracle_VM_VirtualBox_Extension_Pack-4.1.22-80657.vbox-extpack
Download VirtualBox 4.1.22 User Manual (PDF) (4.9MB)
MD5 Sum: 356493319a41d5fa3ff0b1205d5b6b75 UserManual.pdf
Not sure why your replying to me and not the OP however...
avanish wrote: I believe the license of fusion version 5 is backward compatible with older versions. Not sure though.
While VMware has in the past made a newer version serial number of some of its products work in a previous version then released after the fact, nonetheless a VMware Fusion 5 serial number will not work in a previous version of VMware Fusion that is supported under Mac OS X 10.5.8. (I know this since I had already tested it.)
My advise, contact vmware and verify it.
Already said "...although you might want to contact VMware Sales directly."!
Additionally while I'm quite sure if VMware wanted to they could supply a 30-day serial number for say VMware Fusion 3, being the last version that will work on Mac OS X 10.5.8, however I suspect they wouldn't and advise the User it's time to move on and upgrade appropriately and is in part why I supplied the VirtualBox info as I believe at this point in time it's a more viable option with an OS and hardware that is that many generations old then getting a free 30 day serial number for VMware Fusion 3.x at this point in time. Obviously any Intel based Mac that came with Mac OS X Lion installed is not compatible with Mountain Lion and or VMware Fusion 5 so Abcd_012's choices considering the age of the hardware is limited.
@Abcd_012 as I previously said "...although you might want to contact VMware Sales directly." it certainly doesn't hurt to ask and if you do not get anywhere with getting a serial number for VMware Fusion 3.x, the last version that will run under Mac OS X 10.5.8, then you have the VirtualBox info as an alternative.