Yes, there are many accessories (cables and docks) that enable TV-out functionality on the iPhone. You should look to buy them either directly from Apple or from an authorized reseller (it must have the "Works with iPhone" or "Made for iPod" label), given that since firmware 3.0 (that includes your iPhone 3GS) all generic TV-out cables will show you an annoying "This accessory is not suported" warning every few seconds, and that's if they work at all. There are some work arounds for this, but they're not worth discussing in this question.

Using these accessories can only get you so far. Only a handful of applications on the iPhone have built-in TV-out capability (photo and video apps). For the rest of them, at the moment your only choice is to jailbreak your iPhone and then install either iPhone TV-out or ScreenSplitr (via Cydia/Icy). The later has the adventage of having full support to project the iPhone's SpringBoard (the "Home" screen) and any apps, both in the iPhone and your TV/Projector (mirror mode), and that's most probably what you are looking for.

If your iPhone happens to be jailbroken, there is a tweak in Cydia called "Veency" which allows you to use VNC to connect to your iPhone from your computer and just project your computer screen instead. Also, one advantage about Veency is that you can use your computer's keyboard to type on the iPhone.