Here's a fully successful download and extraction with MD5sum at the end.
Code:
sharlow@diablo:~$ wget http://files.zimbra.com/downloads/5.0.2_GA/zcs-5.0.2_GA_1975.DEBIAN4.0.20080130234700.tgz
--11:06:19-- http://files.zimbra.com/downloads/5.0.2_GA/zcs-5.0.2_GA_1975.DEBIAN4.0.20080130234700.tgz
=> `zcs-5.0.2_GA_1975.DEBIAN4.0.20080130234700.tgz'
Resolving files.zimbra.com... 72.21.211.210
Connecting to files.zimbra.com|72.21.211.210|:80... connected.
HTTP request sent, awaiting response... 200 OK
Length: 314,609,607 (300M) [binary/octet-stream]
100%[=============================================>] 314,609,607 178.11K/s ETA 00:00
11:35:42 (174.35 KB/s) - `zcs-5.0.2_GA_1975.DEBIAN4.0.20080130234700.tgz' saved [314609607/314609607]
sharlow@diablo:~$ ls -l | grep zcs
-rw-r--r-- 1 sharlow domainadmins 314609607 2008-01-31 03:53 zcs-5.0.2_GA_1975.DEBIAN4.0.20080130234700.tgz
sharlow@diablo:~$ tar -xzvf zcs-5.0.2_GA_1975.DEBIAN4.0.20080130234700.tgz
zcs-5.0.2_GA_1975.DEBIAN4.0.20080130234700/
zcs-5.0.2_GA_1975.DEBIAN4.0.20080130234700/packages/
zcs-5.0.2_GA_1975.DEBIAN4.0.20080130234700/packages/zimbra-ldap_5.0.2_GA_1975.DEBIAN4.0_i386.deb
zcs-5.0.2_GA_1975.DEBIAN4.0.20080130234700/packages/zimbra-core_5.0.2_GA_1975.DEBIAN4.0_i386.deb
zcs-5.0.2_GA_1975.DEBIAN4.0.20080130234700/packages/zimbra-logger_5.0.2_GA_1975.DEBIAN4.0_i386.deb
zcs-5.0.2_GA_1975.DEBIAN4.0.20080130234700/packages/zimbra-spell_5.0.2_GA_1975.DEBIAN4.0_i386.deb
zcs-5.0.2_GA_1975.DEBIAN4.0.20080130234700/packages/zimbra-proxy_5.0.2_GA_1975.DEBIAN4.0_i386.deb
zcs-5.0.2_GA_1975.DEBIAN4.0.20080130234700/packages/zimbra-mta_5.0.2_GA_1975.DEBIAN4.0_i386.deb
zcs-5.0.2_GA_1975.DEBIAN4.0.20080130234700/packages/zimbra-store_5.0.2_GA_1975.DEBIAN4.0_i386.deb
zcs-5.0.2_GA_1975.DEBIAN4.0.20080130234700/packages/zimbra-apache_5.0.2_GA_1975.DEBIAN4.0_i386.deb
zcs-5.0.2_GA_1975.DEBIAN4.0.20080130234700/packages/zimbra-snmp_5.0.2_GA_1975.DEBIAN4.0_i386.deb
zcs-5.0.2_GA_1975.DEBIAN4.0.20080130234700/docs/
zcs-5.0.2_GA_1975.DEBIAN4.0.20080130234700/docs/ZCS_Toaster.pdf
zcs-5.0.2_GA_1975.DEBIAN4.0.20080130234700/docs/MigrationWizard_Domino.pdf
zcs-5.0.2_GA_1975.DEBIAN4.0.20080130234700/docs/quick_start.pdf
zcs-5.0.2_GA_1975.DEBIAN4.0.20080130234700/docs/zcl.txt
zcs-5.0.2_GA_1975.DEBIAN4.0.20080130234700/docs/zimbra_public_eula_2.1.txt
zcs-5.0.2_GA_1975.DEBIAN4.0.20080130234700/docs/Import_Wizard_Outlook.pdf
zcs-5.0.2_GA_1975.DEBIAN4.0.20080130234700/docs/YPL.txt
zcs-5.0.2_GA_1975.DEBIAN4.0.20080130234700/docs/Zimbra Schema.pdf
zcs-5.0.2_GA_1975.DEBIAN4.0.20080130234700/docs/admin.pdf
zcs-5.0.2_GA_1975.DEBIAN4.0.20080130234700/docs/MigrationWizard.pdf
zcs-5.0.2_GA_1975.DEBIAN4.0.20080130234700/docs/User Instructions for ZCS Import Wizard.pdf
zcs-5.0.2_GA_1975.DEBIAN4.0.20080130234700/docs/OSmultiserverinstall.pdf
zcs-5.0.2_GA_1975.DEBIAN4.0.20080130234700/docs/Zimbra_Release_Note.pdf
zcs-5.0.2_GA_1975.DEBIAN4.0.20080130234700/docs/RNZCSO_2005Beta.pdf
zcs-5.0.2_GA_1975.DEBIAN4.0.20080130234700/data/
zcs-5.0.2_GA_1975.DEBIAN4.0.20080130234700/data/versions-init.sql
zcs-5.0.2_GA_1975.DEBIAN4.0.20080130234700/bin/
zcs-5.0.2_GA_1975.DEBIAN4.0.20080130234700/bin/get_plat_tag.sh
zcs-5.0.2_GA_1975.DEBIAN4.0.20080130234700/bin/zmdbintegrityreport
zcs-5.0.2_GA_1975.DEBIAN4.0.20080130234700/util/
zcs-5.0.2_GA_1975.DEBIAN4.0.20080130234700/util/globals.sh
zcs-5.0.2_GA_1975.DEBIAN4.0.20080130234700/util/modules/
zcs-5.0.2_GA_1975.DEBIAN4.0.20080130234700/util/modules/getconfig.sh
zcs-5.0.2_GA_1975.DEBIAN4.0.20080130234700/util/modules/packages.sh
zcs-5.0.2_GA_1975.DEBIAN4.0.20080130234700/util/modules/postinstall.sh
zcs-5.0.2_GA_1975.DEBIAN4.0.20080130234700/util/addUser.sh
zcs-5.0.2_GA_1975.DEBIAN4.0.20080130234700/util/utilfunc.sh
zcs-5.0.2_GA_1975.DEBIAN4.0.20080130234700/install.sh
zcs-5.0.2_GA_1975.DEBIAN4.0.20080130234700/readme_binary.txt
zcs-5.0.2_GA_1975.DEBIAN4.0.20080130234700/README.txt
zcs-5.0.2_GA_1975.DEBIAN4.0.20080130234700/readme_source.txt
sharlow@diablo:~$ rm -rf zcs-5.0.2_GA_1975.DEBIAN4.0.20080130234700
sharlow@diablo:~$ md5sum zcs-5.0.2_GA_1975.DEBIAN4.0.20080130234700.tgz
7830e138b976305fd6e1bc03a15bb2ce zcs-5.0.2_GA_1975.DEBIAN4.0.20080130234700.tgz Compare the file size (314609607 bytes) and md5sum (7830e138b976305fd6e1bc03a15bb2ce) to what you have and we'll see if your file is good.