Lead source is definately a frequently forgotten tool for marketeers - You can setup your lead sources in the Tools, Manage, Lead Sources menu if you have access to it (you may need to get your system admin to give you permission to access this menu item).
What I've always tried to show in my training courses is the lead source functionality; as while it is simple, it can be a powerful budgetary tool when used correctly.
If you set up a lead source, for example: here in the UK, we have a annual show called the Technology For Marketing show at the NEC in Birmingham, so the lead source would be called something like "TFM 2008". While setting up this lead source you can assign a cost to it, so if you had a stand for example or you were simply sending some staff along, you can put the cost of the stand/expenses as the cost of the lead source (another example would be a purchased or rented marketing list which you have imported into the system).
When you get back from the trade show, you can now start adding all those business cards that you collected while you were there making sure you select the TFM 2008 lead source.
Obviously you will also have people contact you after the event which you can also list as having a lead source of TFM 2008.
Now you can use one of the system standard reports: Contacts by Lead Source
This report shows you a break down of your contacts by lead source and nicely shows you a "cost per lead" based on the number of contacts you have with that lead source and the amount you entered as the cost for the lead source. This report can help you decide whether having a stand at the show was worth it for the number of new contacts you have and therefore whether to go again the following year.
Remember you can also record a lead source for an opportuntiy, so you can then also start reporting on how much potential revenue your trade show is generating year on year. Handy huh!
Good marketing!
Ben