At Admin Panel -> Settings -> Switches, you have an "online users" option which enables a listing of users who are currently online browsing the site. By default online users are shown on the front page, in categories and on thread pages (the latter two just show those currently browsing said category and said thread). You can place an online user list anywhere by making a toplist of online users at Admin -> Themes -> Toplist Generator.
You can configure the online user lists under Admin Panel -> Members -> Member Settings -> "Online user list". This allows you to set how recently the member needs to have clicked something to be considered to be still online, and how long to retain them in the database online user table. It's recommended you keep this at 15 minutes.
To change the date format used in the online user list, go to Admin Panel -> Settings -> Localization.
Global Online User Template Variables
these work anywhere, not just in online user toplists)
{TOTALONLINE} - Total users online.
{TOTALGUESTSONLINE} - Total unregistered users online.
{TOTALMEMBERSONLINE} - Total registered users online.
{TOTALSPIDERSONLINE} - Total search engine spiders online.
{MAXGUESTSONLINE} - Most ever unregistered users online.
{MAXGUESTSONLINEDATE} - The date/time of the most ever unregistered users online.
{MAXMEMBERSONLINE} - Most ever registered users online.
{MAXMEMBERSONLINEDATE} - The date/time of the most ever registered users online.
{MAXTOTALONLINE} - The most overall users online.
{MAXTOTALONLINEDATE} - The date/time of the most overall users online.
Online User Toplist Template Variables
All member variables with {ONLINE in place of {MEMBER, plus
{ONLINEMEMBERID} - The id of the member, or false if a guest.
{ONLINENAME} - THe username of the member.
{ONLINEGROUPNAMESTYLE} - The style class for the usergroup.
{ONLINEIP} - The IP address of the visitor.
{ONLINELOCATION} - The url the user last viewed.
{ONLINEAREANAME} - The name of the location viewed, if available.
{ONLINEDATE} - The time of the user's last action.
{ONLINESTARTDATE} - The time the user came online.
{ONLINEBROWSER} - The user agent of the person.
{ONLINECATURL} - The url of the category being viewed by the online user.
{ONLINECATNAME} - The name of the category being viewed/
{ONLINECATID} - The id of the category being viewed by the online user. False if they're not in a category.
{ONLINETHREADID} - The id of the comments thread being viewed by the online user. 0 if they're not viewing a comments thread.
{ONLINEISROBOT} - True if it's a search engine spider, otherwise false.
{ONLINEREADYTOCHATICON} - When using the instant messaging system, this places a chat icon where appropriate.
{ONLINEINVISMARKER} - A marker to show that the member is invisible, when viewed by those who can view the invisible.
You can also use all category and listing variables whenever {ONLINECATID} and {ONLINETHREADID} are greater than 0, to show info about the category/thread being viewed.