Teamspeak, Ventrilo, or Skype
Nothing beats playing online games with the ability to dynamically talk to your teammates. Of course, nothing also beats teamkilling your teammate and hearing him swear like a sailor on the mic. It's good like that. It's almost a ritual for Ash_Link, Spark Electro, and myself to get on voice chat and fag around on SOME Half Life game. It's just like that. This is necessary.
Some people argue against these and claim that using in-game voice chat is optimal. While this is usually true, if you JUST want to chat with your friends, this is the easiest. After all, you can change servers and still be chatting about it, instead of using an instant messenger to send messages like "GUYS I CANT LOG IN THE ADMIN H8s ME".
If it comes between them, I'd reccomend getting all of them. Skype is a great program for casual chat with friends, but not for gaming. If you're going to voice chat with your girlfriend, this is pretty much the way to do it. Teamspeak and Ventrilo are much more gaming based, which is obvious, because they're as minimalistic as possible. Teamspeak's quality kinda sucks, but Ventrilo requires a lot more bandwidth. It's your call. I have all three.
AOL Instant Messenger, MSN Messenger, Yahoo Messenger, or Xfire
This should be blatantly obvious. These kinda programs are the grown-up children of your basic chat channels, and are absolutely required if you want to keep in touch with anyone at all. Everyone, from 40 year old pedophiles to 12 year old girls have AOL Instant Messenger, MSN, or Yahoo. Personally, I prefer AOL (After I use a mod and make all the ads and shit away), but some people prefer MSN or Yahoo... or even XFire. I have all four. It really depends who your friends are- MSN fags, Yahoo fags, AOL fags, or... yeah, XFire shit.
Oh, yeah, Xfire is probably the best gamer chat client. It's not good for much else other than gaming, but.... whatever. It keeps track of all your popular online games, how much you've played them, who you played them with, what you're playing now, what your friends are playing... all the good stuff. But still, it's not good for much else. I still prefer AIM.
Winamp with the CCP or Kazaa Lite Codec Pack
Windows Media Player blows. Face it. It plays stuff horribly, has all sorts of extraneous shit you don't need, has no useable skins, takes up an unholy amount of desktop space, and just pisses you off. That's why I reccomend Winamp.
Why Winamp?
Because Winamp kicks the llama's ass. Ever since it's existance, it's ruled the media player business because it's simple and upgradeable. I have it set as a taskbar-sized thing right above my taskbar. Some people have it in a little box. Whatever you want, it can do it. Well.
Now, CCP (The Community Codec Pack) or the Kazaa Lite Codec Pack are great packs. Essentially, they take all the guesswork and adaptation of the popular codecs (Matroska, XVid, DivX, etc) and work them out to be one simple install. Of course, Winamp seems to work best with these (And if not, the Kazaa Lite Codec Pack comes with "Media Player Lite" to fix that). Either way, it allows you to play all of those little miscellaneous video files you find laying around.