How can I find out who is connected to my wireless router? (Linksys WRT54g)

Status -> Local Network -> (button) DHCP Clients Table? http://ui.linksys.com/WRT54G/v8.2/8.2.05/StaLan.htm

Another option is to enable wireless mac filtering, then click Edit Mac Filter list, where you can see the MAC on anything connected via wlan.

http://ui.linksys.com/WRT54G/v8.2/8.2.05/WFilter.htm

Type 192.168.1.1 in your address bar and click on Status. Under Status menu, click on Local Network. In Local Network click on DHCP Clients Table. There, are all the computers connected to your Linksys Modem. If No computers connected to you, the DHCP Active IP Table that pop up, will be empty. Experiment connect with another computer and come back to this menu to check. There will be your pc or laptop where Client Host Name is like: Bob's pc, and IP Adress like 192.168..102, Mac Address like 00.09.10.b5.44 and Expites like 23.43.55