I just got Verizon FIOS (15 MB down and 2MB up), fiber to the house!  They actually wired cat5 from the outside to my kitchen from the fiber connection.  So I wanted to have my 3 computers in my office be able to communicate wirelessly to the wireless router that was given to me from Verizon.  I had an old linksys WRT54G and decided to search on hacking this router.  By the way you can buy these for like $50 now if you need one.  I wanted to get my WRT54G running in client mode.  Well what do you know, I pulled up this killer open source software that turns your router into a client access point and a nice firewall.  Unbelievable.  It actually works.  I am going to post how to do it here in case this link goes bad.  Follow the instructions exactly and this will work first time.

Instructions:

1. Download the "wrt54g alchemy firmware" (google this, or go here) version 6rc5 from the Internet (it is the only one compatible with the new models wrt54g V2.2 & wrt54gs V1.1).
2. Upload the firmware to the wrt54g via the "Administration->Firmware Upgrade" option in the router's web interface.
3. Reset the device (press reset button until power led flashes - this could take 20 seconds or more). In the following steps, leave all settings which are not mentioned in the description at their default (only change the ones specifically mentioned in the step).
Note: We will use addresses that end in 128-255 for the wrt54g router and it's attached PCs. To make sure there is no overlap in the addresses assigned by the two routers, we have to make sure that your main router only assign addresses below 128 to its own clients. For example, if your main router address is 192.168.0.1, its DHCP address range could be 192.168.0.50 - 192.168.0.120. Another important note regarding the main router: some routers allow you to disable the broadcast of the SSID, but for this setup to work properly the SSID broadcast MUST BE ENABLED (which is usually the default behavior).
4a. In "Setup::Basic setup" screen, set Internet Connection Type to "Automatic Configuration - DHCP".
4b. Set the local IP to 192.168.0.129, assuming your main router is 192.168.0.x (in general, if your main router is a.b.c.d set the wrt54g to a.b.c.129).
4c. Set the Subnet Mask to 255.255.255.128.
4d. Set the "Gateway" to the IP address of your main router (e.g. 192.168.0.1).
4e. Set the DHCP Server to Enabled, and starting IP Address to something above 129 (e.g. 140).
4f. Save the settings on this page. You should reboot the attached PC, since the subnet mask has changed. Note: from now on you have to use the new local IP you set in step 4b (e.g. http:// 192.168.0.129) to connect to the router from your PC.
5. In "Setup::Advanced Routing" screen, make sure operating mode is "Gateway".
6. In "Wireless::Basic Settings" screen, set Wireless mode to "Client", wireless network mode to "mixed", and SSID to your main wireless router's SSID.
7. In "Wireless::Security" screen, set your WEP/WPA settings which match the main wireless router. You should now be connected to the main wireless router. Note: if you have MAC filtering set up on the main router (which allows only specific clients to connect), then obviously you have to add the client router to the list of allowed clients.
To verify that you've established a connection to the main router, you can check the "Status::Router" screen, which should show an IP Address assigned by the main router (which would end with a number below 128), and the "Status::Wireless" page should show the AP Signal strength together with the mac address of the main router. Also you should now be able to ping the main router and even log in to it (using h ttp://192.168.0.1) from the PC attached to the client router (the wrt54g).
8. In "Security::Firewall" screen, disable firewall protection, as this subnet is already behind your main router's firewall. Also make sure that "Block Anonymous Internet Requests" is unchecked.
9. In "Administration::Management" screen, you can leave all settings at their default. You may want to enable remote management and Telnet or SSHD, especially if you want to be able to log in to the wrt54g from a computer which is not directly attached to it.
10. To enable PCs attached to the main router to be able connect to PCs attached to the wrt54g: Login into the wrt54g using telnet or ssh by running the command "telnet 192.168.0.129" and use same root/passwd as for the web interface. Then type this command (copy it exactly):
# echo 1 > /proc/sys/net/ipv4/conf/`route | grep default | awk '{print $NF}'`/proxy_arp
Now you should be able to ping/telnet to any PC attached to the wrt54g from any PC attached to the main router. Btw, this assumes that the subnet mask of the main router is the default 255.255.255.0.

Important note about the last step : The last step (which is an optional step) allows PCs attached to the main router to be able to connect to PCs attached to the client router, by specyfing the IP of the destination PC, for example "telnet 192.168.0.150". But they are still on two different subnets which do not share their broadcast messages, therefore when browsing PCs on the local network you will not automatically see the PCs attached to the other router. But you can always connect to them by explicitly specyfing the IP. Btw, this step was added after the initial posting of this HOWTO and solves a lot of the issues people discussed later in this thread, so don't worry if you read posts in this thread about problems with communicating between PCs attached to different routers. Also note that this proxy_arp setting is not saved in the WRT54G non-volatile memory like all the other settings, so when the router is rebooted (like after a power outage), it will be cleared and you will need to repeat step 10 to set the proxy_arp back on.
Done !!! That's it !!!!