For any Cisco engineer or for that matter any network engineer or student, the following tools are of great help. All these tools are free and their latest versions can be downloaded from their respective publisher's / distributor's website by clicking on the icons

This free program allows users to build network topologies to test the functionality of IOS on a desktop PC, without the need of an actual physical Cisco device. It supports different networking medium such as Ethernet, Serial link, ATM, and POS interfaces for the 1700, 2600, 3600, 3700, and 7200 hardware platforms.

Tftpd32 is a free, open source application which includes DHCP, TFTP, DNS, SNTP and Syslog servers as well as a TFTP client. The TFTP client and server are fully compatible with TFTP option support (tsize, blocksize and timeout), which allow maximum performance when transferring data

Wireshark is a free and open-source packet analyzer. It is used for network troubleshooting and analysis

This Advanced IP Address Calculator is a freeware Windows 95/98/NT/2000/XP utility that lets you calculate every aspect of your subnet configuration. It is very useful for any network engineer or student who needs a fast way to determine subnets.

PuTTY is a free and open source terminal emulator application which acts as a client for the SSH, Telnet and rlogin protocols and as a serial console client.

TfGen allows you to generate traffic on the local area network, e.g. ethernet, to allow evaluation of network performance.

Mcast allows your PC to receive multicast traffic on the local area network, e.g. ethernet. Use this multicast traffic receiver with TfGen.